REFRIGERATION APPARATUS TECHNICAL FIELD The present invention relates to a refrigeration apparatus, and particularly relates 5 to a refrigeration apparatus which has a refrigerant circuit configured to be capable of switching between a cooling operation and a heating operation and which performs a multistage compression refrigeration cycle. BACKGROUND ART As one conventional example of a refrigeration apparatus which has a 10 refrigerant circuit configured to be capable of switching between a cooling operation and a heating operation and which performs a multistage compression refrigeration cycle, there is disclosed in Patent Document I an air-conditioning apparatus which has a refrigerant circuit configured to be capable of switching between an air-cooling operation and an air-warming operation and which performs a two-stage compression 15 refrigeration cycle. This air-conditioning apparatus primarily has a compressor having two compression elements connected in series, a four-way switching valve for switching between an air-cooling operation and an air-warming operation, an outdoor heat exchanger, and an indoor heat exchanger. <Patent Document 1> 20 Japanese Laid-open Patent Application No. 2007-232263 Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of 25 these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application. Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated 30 element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. DISCLOSURE OF THE INVENTION According to the present invention, there is provided a refrigeration apparatus 35 including a compression mechanism, a heat source-side heat exchanger which functions as a radiator or evaporator of refrigerant, a usage-side heat exchanger which functions as an evaporator or radiator of refrigerant, a switching mechanism, and an intermediate heat exchanger. The compression mechanism has a plurality of compression elements and is configured so that the refrigerant discharged from the first-stage compression element, which is one of a plurality of compression elements, is sequentially 5 compressed by the second-stage compression element. As used herein, the term ''compression mechanism" refers to a compressor in which a plurality of compression elements are integrally incorporated, or a configuration that includes a compressor in which a single compression element is incorporated and/or a plurality of compressor in which a plurality of compression elements have been incorporated are connected 10 together. The phrase "the refrigerant discharged from a first-stage compression element, which is one of the plurality of compression elements, is sequentially compressed by a second-stage compression element" does not mean merely that two compression elements connected in series are included, namely, the "first-stage compression element" and the "second-stage compression element," but that a plurality 15 of compression elements are connected in series and the relationship between the compression elements is the same as the relationship between the aforementioned "first-stage compression element" and "second-stage compression element." The switching mechanism is a mechanism for switching between a cooling operation state wherein refrigerant is circulated sequentially through the compression mechanism, the 20 heat source-side heat exchanger functioning as a refrigerant radiator, and the usage-side heat exchanger functioning as an evaporator of refrigerant; and a heating operation state wherein refrigerant is circulated sequentially through the compression mechanism, the usage-side heat exchanger functioning as a refrigerant radiator, and the heat source side heat exchanger functioning as an evaporator of refrigerant. The intermediate heat 25 exchanger is a heat exchanger capable of functioning as a cooler of refrigerant discharged from the first-stage compression element and drawn into the second-stage compression element when the switching mechanism has been set to the cooling operation state, and also capable of functioning as an evaporator of refrigerant whose heat is radiated in the usage-side heat exchanger in parallel with the heat source-side 30 heat exchanger when the switching mechanism has been set to the heating operation state. In a conventional air-conditioning apparatus, since the refrigerant discharged from a first stage compression element of the compressor is drawn into a second-stage compression element of the compressor and further compressed, the temperature of the 35 refrigerant discharged from the second-stage compression element of the compressor increases. In an outdoor heat exchanger functioning as a refrigerant radiator, for example, the temperature difference between the refrigerant and the water and/or air as 2 a heating source increases, and heat radiation loss in the outdoor heat exchanger increases, therefore causing a problem in that high operation efficiency is difficult to obtain. As a countermeasure to this problem, in cases in which an intermediate heat 5 exchanger is provided which functions as a cooler of refrigerant discharged from the first-stage compression element and drawn into the second-stage compression element, such as is the case with this refrigeration apparatus, the temperature of the refrigerant drawn into the second-stage compression element is lower, and the temperature of the refrigerant ultimately discharged from the compression mechanism can therefore be 10 kept low in comparison with cases in which no intermediate heat exchanger is provided. Operation efficiency can therefore be improved during the cooling operation because heat radiation loss during the cooling operation can be reduced in the heat source-side heat exchanger which functions as a refrigerant radiator. However, since an intermediate heat exchanger is provided, the heat that would 15 have been useable in the usage-side heat exchanger during the heating operation if there were no intermediate heat exchanger functioning as a cooler of refrigerant discharged from the first-stage compression element and drawn into the second-stage compression element during the heating operation is radiated to the exterior from the intermediate heat exchanger, whereby the heating capacity in the usage-side heat exchanger 20 decreases, and the operation efficiency during the heating operation decreases. As a countermeasure to this, for example, an intermediate heat exchanger bypass tube for bypassing the intermediate heat exchanger is provided, and during the heating operation the intermediate heat exchanger bypass tube is used so that the refrigerant discharged from the first-stage compression element and drawn into the second-stage 25 compression element bypasses the intermediate heat exchanger so as not to be cooled, thereby creating a state in which the intermediate heat exchanger is not used, whereby the loss of heating capacity in the usage-side heat exchanger is minimized during the heating operation, and operation efficiency during the heating operation can be prevented from decreasing. 30 However, when a state is created in which the intermediate heat exchanger is not used during the heating operation, the intermediate heat exchanger is provided as a heat exchanger used only during the cooling operation, and the intermediate heat exchanger will therefore be a device not used during the heating operation. In view of this, in the refrigeration apparatus disclosed herein, the intermediate 35 heat exchanger is made to function as a cooler when the switching mechanism has been set to the cooling operation state, and when the switching mechanism has been set to the heating operation state, the intermediate heat exchanger is made to function as an evaporator of refrigerant whose heat is radiated in the usage-side heat exchanger. Therefore, in this refrigeration apparatus, the temperature of the refrigerant discharged from the compression mechanism can be minimized during the cooling operation, and 5 during the heating operation, the refrigerant evaporation capacity can be improved while the heat radiated to the exterior from the intermediate heat exchanger can be minimized. Thereby, in the refrigeration apparatus disclosed herein, during the cooling operation, heat radiation loss in the heat source-side heat exchanger functioning as a 10 refrigerant radiator is reduced, and the operation efficiency during the cooling operation can be improved, while during the heating operation, it is possible to efficiently use the intermediate heat exchanger, loss of heating capacity in the usage side heat exchanger can be minimized, and the operation efficiency during the heating operation can be prevented from decreasing. 15 In an embodiment of the invention, the intermediate heat exchanger is provided with an intermediate refrigerant tube for drawing the refrigerant discharged from the first-stage compression element into the second-stage compression element, an intermediate heat exchanger bypass tube is connected to the intermediate refrigerant tube so as to bypass the intermediate heat exchanger; and the refrigeration apparatus 20 further comprises an intake return tube for connecting one end of the intermediate heat exchanger with an intake side of the compression mechanism, and an intermediate heat exchanger return tube for connecting the other end of the intermediate heat exchanger with the portion between the usage-side heat exchanger and the heat source-side heat exchanger. 25 In the embodiment, the intermediate-pressure refrigerant flowing through the intermediate refrigerant tube can be cooled by the intermediate heat exchanger during the cooling operation, and during the heating operation, the intermediate-pressure refrigerant flowing through the intermediate refrigerant tube is made to bypass the intermediate heat exchanger by the intermediate heat exchanger bypass tube, and some 30 of the refrigerant cooled in the usage-side heat exchanger can be drawn into and evaporated in the intermediate heat exchanger and can be returned to the intake side of the compression mechanism by the intake return tube and the intermediate heat exchanger return tube. At the start of the operation for setting the switching mechanism to the cooling 35 operation state, the refrigerant discharged from the first-stage compression element may be drawn into the second-stage compression element through the intermediate heat exchanger bypass tube, and the intermediate heat exchanger may be connected with the intake side of the compression mechanism through the intake return tube. In an embodiment of the present invention, at the start of the operation for setting the switching mechanism to the cooling operation state, since the refrigerant 5 discharged from the first-stage compression element is drawn into the second-stage compression element through the intermediate heat exchanger bypass tube and the intermediate heat exchanger is connected with the intake side of the compression mechanism through the intake return tube, even if liquid refrigerant accumulates in the intermediate heat exchanger at the start of the operation for setting the switching 10 mechanism to the cooling operation state, this liquid refrigerant can be removed from the intermediate heat exchanger. Thereby, at the start of the operation for setting the switching mechanism to the cooling operation state, it is possible to avoid states in which liquid refrigerant has accumulated in the intermediate heat exchanger, and the refrigerant discharged from the first-stage compression element can be drawn into the 15 second-stage compression element through the intermediate heat exchanger without any liquid compression occurring in the second-stage compression element as a result of liquid refrigerant accumulating in the intermediate heat exchanger. The intermediate heat exchanger return tube may be provided with a flow rate control valve. 20 In an embodiment of the present invention, refrigerant can be prevented from flowing into the intermediate heat exchanger return tube during the cooling operation, and it is possible to achieve a reliable distribution between the flow rate of the refrigerant flowing through the heat source-side heat exchanger and the flow rate of the refrigerant flowing through the intermediate heat exchanger during the heating 25 operation. An expansion device for isentropically expanding the refrigerant flowing between the heat source-side heat exchanger and the usage-side heat exchanger may be connected to the portion between the heat source-side heat exchanger and the usage side heat exchanger via a rectifier circuit which rectifies the refrigerant flow so that 30 refrigerant flows in from the inlet of the expansion device both in cases in which refrigerant flows from the heat source-side heat exchanger to the usage-side heat exchanger and cases in which refrigerant flows from the usage-side heat exchanger to the heat source-side heat exchanger. In an embodiment of the invention, it is possible to improve the coefficient of 35 performance and to recover energy through the expansion device during both the cooling operation and the heating operation, and operation efficiency during both the cooling operation and the heating operation can therefore be further improved. A gas-liquid separator for performing gas-liquid separation of the refrigerant may be connected to an outlet of the expansion device, and a second-stage injection 5 tube for returning to the second-stage compression element gas refrigerant separated in the gas-liquid separator is connected to the gas-liquid separator. In an embodiment of the present invention, operation efficiency can be further improved because intermediate pressure injection can be performed for returning intermediate-pressure refrigerant to the second-stage compression element. BRIEF DESCRIPTION OF THE DRAWINGS FIG 1 is a schematic structural diagram of an air-conditioning apparatus as an 5 embodiment of the refrigeration apparatus according to the present invention. FIG 2 is a diagram showing the flow of refrigerant within the air-conditioning apparatus during the air-cooling operation. FIG 3 is a pressure-enthalpy graph representing the refrigeration cycle during the air-cooling operation. 10 FIG 4 is a temperature-entropy graph representing the refrigeration cycle during the air-cooling operation. FIG 5 is a diagram showing the flow of refrigerant within the air-conditioning apparatus during the air-warming operation. FIG 6 is a pressure-enthalpy graph representing the refrigeration cycle during the 15 air-warming operation. FIG 7 is a temperature-entropy graph representing the refrigeration cycle during the air-warming operation. FIG 8 is a flowchart of the air-cooling start control. FIG 9 is a diagram showing the flow of refrigerant within the air-conditioning 20 apparatus during the air-cooling start control. FIG 10 is a schematic structural diagram of an air-conditioning apparatus according to Modification 1. FIG 11 is an external perspective view of a heat source unit (the fan grill having been removed). 25 FIG 12 is a side view of the heat source unit in which the right plate of the heat source unit has been removed. FIG 13 is a diagram showing the characteristics of the heat transfer coefficient when carbon dioxide of an intermediate pressure lower than the critical pressure is flowing into the heat transfer channel, and of the heat transfer coefficient when carbon dioxide of a high 30 pressure exceeding the critical pressure is flowing into the heat transfer channel. FIG 14 is a schematic structural diagram of an air-conditioning apparatus according to Modification 3. FIG 15 is a pressure-enthalpy graph representing the refrigeration cycle during the air-cooling operation in the air-conditioning apparatus according to Modification 3. 6 FIG 16 is a temperature-entropy graph representing the refrigeration cycle during the air-cooling operation in the air-conditioning apparatus according to Modification 3. FIG 17 is a pressure-enthalpy graph representing the refrigeration cycle during the air-warming operation in the air-conditioning apparatus according to Modification 3. 5 FIG 18 is a temperature-entropy graph representing the refrigeration cycle during the air-warming operation in the air-conditioning apparatus according to Modification 3. FIG 19 is a schematic structural diagram of an air-conditioning apparatus according to Modification 4. FIG 20 is a pressure-enthalpy graph representing the refrigeration cycle during the 10 air-warming operation in the air-conditioning apparatus according to Modification 4. FIG 21 is a temperature-entropy graph representing the refrigeration cycle during the air-warming operation in the air-conditioning apparatus according to Modification 4. FIG 22 is a schematic structural diagram of an air-conditioning apparatus according to Modification 5. 15 FIG 23 is a pressure-enthalpy graph representing the refrigeration cycle during the air-cooling operation in the air-conditioning apparatus according to Modification 5. FIG 24 is a temperature-entropy graph representing the refrigeration cycle during the air-cooling operation in the air-conditioning apparatus according to Modification 5. FIG 25 is a schematic structural diagram of an air-conditioning apparatus according 20 to Modification 6. FIG 26 is a schematic structural diagram of an air-conditioning apparatus according to Modification 7. FIG 27 is a schematic structural diagram of an air-conditioning apparatus according to Modification 8. 25 FIG 28 is a schematic structural diagram of an air-conditioning apparatus according to Modification 9. FIG 29 is a schematic structural diagram of an air-conditioning apparatus according to Modification 10. FIG 30 is a schematic structural diagram of an air-conditioning apparatus according 30 to Modification 11. FIG 31 is a schematic structural diagram of an air-conditioning apparatus according to Modification 11. FIG 32 is a schematic structural diagram of an air-conditioning apparatus according to Modification 12. 7 FIG 33 is a schematic structural diagram of an air-conditioning apparatus according to Modification 13. EXPLANATION OF THE REFERENCE NUMERALS 1 Air-conditioning apparatus (refrigeration apparatus) 5 2, 102, 202, 302 Compression mechanism 3 Switching mechanism 4 Heat source-side heat exchanger 6 Usage-side heat exchanger 7, 307 Intermediate heat exchanger 10 8, 308 Intermediate refrigerant tube 9, 309 Intermediate heat exchanger bypass tube 92, 392 Second intake return tube 94, 394 Intermediate heat exchanger return tube 94b, 394b Intermediate heat exchanger return valve (flow rate control valve) 15 97 Expansion device 17 Rectifier circuit (bridge circuit) 18 Receiver (gas-liquid separator) 18c Second second-stage injection tube BEST MODE FOR CARRYING OUT THE INVENTION 20 Embodiments of the refrigeration apparatus according to the present invention are described hereinbelow with reference to the drawings. (1) Configuration of air-conditioning apparatus FIG 1 is a schematic structural diagram of an air-conditioning apparatus 1 as an embodiment of the refrigeration apparatus according to the present invention. The air 25 conditioning apparatus 1 has a refrigerant circuit 10 configured to be capable of switching between an air-cooling operation and an air-warming operation, and the apparatus performs a two-stage compression refrigeration cycle by using a refrigerant (carbon dioxide in this case) for operating in a supercritical range. The refrigerant circuit 10 of the air-conditioning apparatus 1 primarily has a 30 compression mechanism 2, a switching mechanism 3, a heat source-side heat exchanger 4, a bridge circuit 17, a receiver 18, a first expansion mechanism 5a, a second expansion mechanism 5b, a usage-side heat exchanger 6, and an intermediate heat exchanger 7. In the present embodiment, the compression mechanism 2 is configured from a compressor 21 which uses two compression elements to subject a refrigerant to two-stage 8 compression. The compressor 21 has a hermetic structure in which a compressor drive motor 21 b, a drive shaft 21 c, and compression elements 2c, 2d are housed within a casing 21 a. The compressor drive motor 21b is linked to the drive shaft 21c. The drive shaft 21c is linked to the two compression elements 2c, 2d. Specifically, the compressor 21 has a so 5 called single-shaft two-stage compression structure in which the two compression elements 2c, 2d are linked to a single drive shaft 21c and the two compression elements 2c, 2d are both rotatably driven by the compressor drive motor 21b. In the present embodiment, the compression elements 2c, 2d are rotary elements, scroll elements, or another type of positive displacement compression element. The compressor 21 is configured so as to admit 10 refrigerant through an intake tube 2a, to discharge this refrigerant to an intermediate refrigerant tube 8 after the refrigerant has been compressed by the compression element 2c, to draw the refrigerant discharged to the intermediate refrigerant tube 8 into the compression element 2d, and to discharge the refrigerant to a discharge tube 2b after the refrigerant has been further compressed. The intermediate refrigerant tube 8 is a refrigerant tube for 15 drawing refrigerant into the compression element 2d connected to the second-stage side of the compression element 2c after the refrigerant has been discharged at an intermediate pressure in the refrigeration cycle from the compression element 2c connected to the first stage side of the compression element 2d. The discharge tube 2b is a refrigerant tube for feeding refrigerant discharged from the compression mechanism 2 to the switching 20 mechanism 3, and the discharge tube 2b is provided with an oil separation mechanism 41 and a non-return mechanism 42. The oil separation mechanism 41 is a mechanism for separating refrigerator oil accompanying the refrigerant from the refrigerant discharged from the compression mechanism 2 and returning the oil to the intake side of the compression mechanism 2, and the oil separation mechanism 41 has primarily an oil separator 41a for 25 separating refrigerator oil accompanying the refrigerant from the refrigerant discharged from the compression mechanism 2, and an oil return tube 41b connected to the oil separator 41a for returning the refrigerator oil separated from the refrigerant to the intake tube 2a of the compression mechanism 2. The oil return tube 41b is provided with a pressure-reducing mechanism 41c for depressurizing the refrigerator oil flowing through the oil return tube 41b. 30 A capillary tube is used for the pressure-reducing mechanism 41c in the present embodiment. The non-return mechanism 42 is a mechanism for allowing the flow of refrigerant from the discharge side of the compression mechanism 2 to the heat source-side heat exchanger 4 as a radiator and for blocking the flow of refrigerant from the heat source-side heat exchanger 4 as a radiator to the discharge side of the compression mechanism 2, and a non-return valve is 9 used in the present embodiment. Thus, in the present embodiment, the compression mechanism 2 has two compression elements 2c, 2d and is configured so that among these compression elements 2c, 2d, refrigerant discharged from the first-stage compression element is compressed in 5 sequence by the second-stage compression element. The switching mechanism 3 is a mechanism for switching the direction of refrigerant flow in the refrigerant circuit 10. In order to allow the heat source-side heat exchanger 4 to function as a refrigerant radiator compressed by the compression mechanism 2 and to allow the usage-side heat exchanger 6 to function as an evaporator of refrigerant cooled in the heat 10 source-side heat exchanger 4 during the air-cooling operation, the switching mechanism 3 is capable of connecting the discharge side of the compression mechanism 2 and one end of the heat source-side heat exchanger 4 and also connecting the intake side of the compressor 21 and the usage-side heat exchanger 6 (refer to the solid lines of the switching mechanism 3 in FIG 1, this state of the switching mechanism 3 is hereinbelow referred to as the "cooling 15 operation state"). In order to allow the usage-side heat exchanger 6 to function as a refrigerant radiator compressed by the compression mechanism 2 and to allow the heat source-side heat exchanger 4 to function as an evaporator of refrigerant cooled in the usage side heat exchanger 6 during the air-warming operation, the switching mechanism 3 is capable of connecting the discharge side of the compression mechanism 2 and the usage-side 20 heat exchanger 6 and also of connecting the intake side of the compression mechanism 2 and one end of the heat source-side heat exchanger 4 (refer to the dashed lines of the switching mechanism 3 in FIG 1, this state of the switching mechanism 3 is hereinbelow referred to as the "heating operation state"). In the present embodiment, the switching mechanism 3 is a four-way switching valve connected to the intake side of the compression mechanism 2, the 25 discharge side of the compression mechanism 2, the heat source-side heat exchanger 4, and the usage-side heat exchanger 6. The switching mechanism 3 is not limited to a four-way switching valve, and may be configured so as to have a function for switching the direction of the flow of the refrigerant in the same manner as described above by using, e.g., a combination of a plurality of electromagnetic valves. 30 Thus, focusing solely on the compression mechanism 2, the heat source-side heat exchanger 4, and the usage-side heat exchanger 6 constituting the refrigerant circuit 10; the switching mechanism 3 is configured so as to be capable of switching between the cooling operation state in which refrigerant is circulated in sequence through the compression mechanism 2, the heat source-side heat exchanger 4 functioning as a radiator of the 10 refrigerant, and the usage-side heat exchanger 6 functioning as an evaporator of the refrigerant; and the heating operation state in which refrigerant is circulated in sequence through the compression mechanism 2, the usage-side heat exchanger 6 functioning as a radiator of the refrigerant, and the heat source-side heat exchanger 4 functioning as an 5 evaporator of the refrigerant. The heat source-side heat exchanger 4 is a heat exchanger that functions as a radiator or an evaporator of refrigerant. One end of the heat source-side heat exchanger 4 is connected to the switching mechanism 3, and the other end is connected to the first expansion mechanism 5a via the bridge circuit 17. Though not shown in the drawings, the heat source 10 side heat exchanger 4 is supplied with water or air as a cooling source for conducting heat exchange with the refrigerant flowing through the heat source-side heat exchanger 4. The bridge circuit 17 is disposed between the heat source-side heat exchanger 4 and the usage-side heat exchanger 6, and is connected to a receiver inlet tube 18a connected to the inlet of the receiver 18 and to a receiver outlet tube 18b connected to the outlet of the receiver 15 18. The bridge circuit 17 has four non-return valves 17a, 17b, 17c, 17d in the present embodiment. The inlet non-return valve 17a is a non-return valve that allows only the flow of refrigerant from the heat source-side heat exchanger 4 to the receiver inlet tube 18a. The inlet non-return valve 17b is a non-return valve that allows only the flow of refrigerant from the usage-side heat exchanger 6 to the receiver inlet tube 18a. In other words, the inlet non 20 return valves 17a, 17b have a function for allowing refrigerant to flow from one among the heat source-side heat exchanger 4 and the usage-side heat exchanger 6 to the receiver inlet tube 18a. The outlet non-return valve 17c is a non-return valve that allows only the flow of refrigerant from the receiver outlet tube 18b to the usage-side heat exchanger 6. The outlet non-return valve 17d is a non-return valve that allows only the flow of refrigerant from the 25 receiver outlet tube 18b to the heat source-side heat exchanger 4. In other words, the outlet non-return valves 17c, 17d have a function for allowing refrigerant to flow from the receiver outlet tube 18b to the heat source-side heat exchanger 4 or the usage-side heat exchanger 6. The first expansion mechanism 5a is a mechanism for depressurizing the refrigerant, is provided to the receiver inlet tube 18a, and is an electrically driven expansion valve in the 30 present embodiment. In the present embodiment, the first expansion mechanism 5a depressurizes the high-pressure refrigerant cooled in the heat source-side heat exchanger 4 to a nearly saturated pressure before feeding the refrigerant to the usage-side heat exchanger 6 via the receiver 18 during the air-cooling operation, and depressurizes the high-pressure refrigerant cooled in the usage-side heat exchanger 6 to a nearly saturated pressure before 11 feeding the refrigerant to the heat source-side heat exchanger 4 via the receiver 18 during the air-warming operation. The receiver 18 is a container provided in order to temporarily accumulate refrigerant that has been depressurized by the first expansion mechanism 5a, so that it is 5 possible to collect excess refrigerant which may be produced depending on operation states in which the quantity of refrigerant circulated in the refrigerant circuit 10 differs between the air-cooling operation and the air-warming operation, for example. The inlet of the receiver 18 is connected to the receiver inlet tube 18a and the outlet is connected to the receiver outlet tube 18b. Also connected to the receiver 18 is a first intake return tube 18f capable of 10 withdrawing refrigerant from inside the receiver 18 and returning the refrigerant to the intake tube 2a of the compression mechanism 2 (i.e., to the intake side of the compression element 2c on the first-stage side of the compression mechanism 2). This first intake return tube 18f is provided with a first intake return on/off valve 18g. The first intake return on/off valve 18g is an electromagnetic valve in the present embodiment. 15 The second expansion mechanism 5b is a mechanism provided to the receiver outlet tube 18b and used for depressurizing the refrigerant, and is an electrically driven expansion valve in the present embodiment. In the present embodiment, in the second expansion mechanism 5b, the refrigerant depressurized by the first expansion mechanism 5a is further depressurized during the air-cooling operation to a low pressure in the refrigeration cycle 20 prior to being fed to the usage-side heat exchanger 6 via the receiver 18, and the refrigerant depressurized by the first expansion mechanism 5a is further depressurized during the air warming operation to a low pressure in the refrigeration cycle prior to being sent to the heat source-side heat exchanger 4 via the receiver 18. The usage-side heat exchanger 6 is a heat exchanger that functions as an evaporator 25 or refrigerant radiator. One end of the usage-side heat exchanger 6 is connected to the first expansion mechanism 5a via the bridge circuit, and the other end is connected to the switching mechanism 3. Though not shown in the drawings, the usage-side heat exchanger 6 is supplied with water or air as a heating source for conducting heat exchange with the refrigerant flowing through the usage-side heat exchanger 6. 30 Thus, in the present embodiment, when the switching mechanism 3 is brought to the cooling operation state by the bridge circuit 17, the receiver 18, the receiver inlet tube 18a, and the receiver outlet tube 18b, the high-pressure refrigerant cooled in the heat source-side heat exchanger 4 can be fed to the usage-side heat exchanger 6 through the inlet non-return valve 17a of the bridge circuit 17, the first expansion mechanism 5a of the receiver inlet tube 12 18a, the receiver 18, the second expansion mechanism 5b of the receiver outlet tube 18b, and the outlet non-return valve 17c of the bridge circuit 17. When the switching mechanism 3 is brought to the heating operation state, the high-pressure refrigerant cooled in the usage-side heat exchanger 6 can be fed to the heat source-side heat exchanger 4 through the inlet non 5 return valve 17b of the bridge circuit 17, the first expansion mechanism 5a of the receiver inlet tube 18a, the receiver 18, the second expansion mechanism 5b of the receiver outlet tube 18b, and the outlet non-return valve 17d of the bridge circuit 17. The intermediate heat exchanger 7 is provided to the intermediate refrigerant tube 8, and is either a cooler of the refrigerant discharged from the first-stage compression element 10 2c and drawn into the compression element 2d, or a heat exchanger capable of functioning as an evaporator of the refrigerant whose heat has been radiated in the usage-side heat exchanger 6. Though not shown in the drawings, the intermediate heat exchanger 7 is supplied with water or air as a cooling source for conducting heat exchange with the refrigerant flowing through the intermediate heat exchanger 7. Thus, it is acceptable to say 15 that the intermediate heat exchanger 7 is a cooler that uses an external heating source, meaning that the intermediate heat exchanger does not use the refrigerant that circulates through the refrigerant circuit 10. An intermediate heat exchanger bypass tube 9 is connected to the intermediate refrigerant tube 8 so as to bypass the intermediate heat exchanger 7. This intermediate heat 20 exchanger bypass tube 9 is a refrigerant tube for limiting the flow rate of refrigerant flowing through the intermediate heat exchanger 7. The intermediate heat exchanger bypass tube 9 is provided with an intermediate heat exchanger bypass on/off valve 11. The intermediate heat exchanger bypass on/off valve 11 is an electromagnetic valve in the present embodiment. Excluding cases in which temporary operations such as the hereinafter-described air-cooling 25 start control are performed, in the present embodiment the intermediate heat exchanger bypass on/off valve 11 is essentially controlled so as to close when the switching mechanism 3 is set for the cooling operation state, and to open when the switching mechanism 3 is set for the heating operation state. In other words, the intermediate heat exchanger bypass on/off valve 11 is closed when the air-cooling operation is performed and opened when the air 30 warming operation is performed. In the intermediate refrigerant tube 8, an intermediate heat exchanger on/off valve 12 is provided to the portion extending from the connection with the end of the intermediate heat exchanger bypass tube 9 near the first-stage compression element 2c to the end of the intermediate heat exchanger 7 near the first-stage compression element 2c. This 13 intermediate heat exchanger on/off valve 12 is a mechanism for limiting the flow rate of refrigerant flowing through the intermediate heat exchanger 7. The intermediate heat exchanger on/off valve 12 is an electromagnetic valve in the present embodiment. Excluding cases in which temporary operations such as the hereinafter-described air-cooling 5 start control are performed, in the present embodiment the intermediate heat exchanger on/off valve 12 is essentially controlled so as to open when the switching mechanism 3 is set for the cooling operation state, and to close when the switching mechanism 3 is set for the heating operation state. In other words, the intermediate heat exchanger on/off valve 12 is controlled so as to open when the air-cooling operation is performed and close when the air 10 warming operation is performed. The intermediate refrigerant tube 8 is also provided with a non-return mechanism 15 for allowing refrigerant to flow from the discharge side of the first-stage compression element 2c to the intake side of the second-stage compression element 2d and for blocking the refrigerant from flowing from the intake side of the second-stage compression element 2d 15 to the discharge side of the first-stage compression element 2c. The non-return mechanism 15 is a non-return valve in the present embodiment. In the present embodiment, the non return mechanism 15 is provided to the intermediate refrigerant tube 8 in the portion extending from the end of the intermediate heat exchanger 7 near the second-stage compression element 2d to the connection with the end of the intermediate heat exchanger 20 bypass tube 9 near the second-stage compression element 2d. Furthermore, a second intake return tube 92 is connected to one end of the intermediate heat exchanger 7 (here, the end near the first-stage compression element 2c), and an intermediate heat exchanger return tube 94 is connected to the other end of the intermediate heat exchanger 7 (herein, the end near the second-stage compression element 25 2d). This second intake return tube 92 is a refrigerant tube for connecting one end of the intermediate heat exchanger 7 and the intake side of the compressor 2 (here, the intake tube 2a) during a state in which the refrigerant discharged from the first-stage compression element 2c is being drawn into the second-stage compression element 2d through the intermediate heat exchanger bypass tube 9. The intermediate heat exchanger return tube 94 30 is a refrigerant tube for connecting the portion between the usage-side heat exchanger 6 and the heat source-side heat exchanger 4 (here, the portion between the second expansion mechanism 5b which depressurizes the refrigerant to a low pressure in the refrigeration cycle and the heat source-side heat exchanger 4 as an evaporator) with the other end of the intermediate heat exchanger 7, when the refrigerant discharged from the first-stage 14 compression element 2c is being drawn into the second-stage compression element 2d through the intermediate heat exchanger bypass tube 9 and the switching mechanism 3 has been set to the heating operation state. In the present embodiment, the second intake return tube 92 is connected at one end to the portion of the intermediate refrigerant tube 8 extending 5 from the connection with the end of the intermediate heat exchanger bypass tube 9 near the first-stage compression element 2c to the end of the intermediate heat exchanger 7 near the first-stage compression element 2c, while the other end is connected to the intake side of the compressor 2 (here, the intake tube 2a). One end of the intermediate heat exchanger return tube 94 is connected to the portion extending from the second expansion mechanism 5b to the 10 heat source-side heat exchanger 4, while the other end is connected to the portion of the intermediate refrigerant tube 8 extending from the end of the intermediate heat exchanger 7 near the first-stage compression element 2c to the non-return mechanism 15. The second intake return tube 92 is also provided with a second intake return on/off valve 92a, and the intermediate heat exchanger return tube 94 is provided with an intermediate heat exchanger 15 return on/off valve 94a. The second intake return on/off valve 92a and the intermediate heat exchanger return on/off valve 94a are electromagnetic valves in the present embodiment. Excluding cases in which temporary operations such as the hereinafter-described air-cooling start control are performed, in the present embodiment the second intake return on/off valve 92a is essentially controlled so as to close when the switching mechanism 3 is set for the 20 cooling operation state, and to open when the switching mechanism 3 is set for the heating operation state. The intermediate heat exchanger return on/off valve 94a is controlled so as to close when the switching mechanism 3 is set for the cooling operation state and to open when the switching mechanism 3 is set for the heating operation state, including cases in which temporary operations such as the hereinafter-described air-cooling start control are 25 performed. Thus, in the present embodiment, owing primarily to the intermediate heat exchanger bypass tube 9, the second intake return tube 92, and the intermediate heat exchanger return tube 94, the intermediate-pressure refrigerant flowing through the intermediate refrigerant tube 8 can be cooled by the intermediate heat exchanger 7 during the 30 air-cooling operation; and during the air-warming operation, the intermediate-pressure refrigerant flowing through the intermediate refrigerant tube 8 can be made to bypass the intermediate heat exchanger 7 by the intermediate heat exchanger bypass tube 9, and some of the refrigerant cooled in the usage-side heat exchanger 6 can be drawn into and evaporated in the intermediate heat exchanger 7 and returned to the intake side of the compression 15 mechanism 2 by the second intake return tube 92 and the intermediate heat exchanger return tube 94. Furthermore, though not shown in the drawings, the air-conditioning apparatus 1 has a controller for controlling the actions of the components constituting the air-conditioning 5 apparatus 1, including the compression mechanism 2, the switching mechanism 3, the expansion mechanisms 5a, 5b, the intermediate heat exchanger bypass on/off valve 11, the intermediate heat exchanger on/off valve 12, the first intake return on/off valve 18g, the second intake return on/off valve 92a, the intermediate heat exchanger return on/off valve 94a, and the like. 10 (2) Action of the air-conditioning apparatus Next, the action of the air-conditioning apparatus 1 of the present embodiment will be described using FIGS. I through 9. FIG 2 is a diagram showing the flow of refrigerant within the air-conditioning apparatus 1 during the air-cooling operation, FIG 3 is a pressure enthalpy graph representing the refrigeration cycle during the air-cooling operation, FIG 4 is 15 a temperature-entropy graph representing the refrigeration cycle during the air-cooling operation, FIG 5 is a diagram showing the flow of refrigerant within the air-conditioning apparatus 1 during the air-warming operation, FIG. 6 is a pressure-enthalpy graph representing the refrigeration cycle during the air-warming operation, FIG 7 is a temperature entropy graph representing the refrigeration cycle during the air-warming operation, FIG 8 is 20 a flowchart of air-cooling start control, and FIG. 9 is a diagram showing the flow of refrigerant within the air-conditioning apparatus 1 during air-cooling start control. Operation control and air-cooling start control during the following air-cooling operation and air-warming operation are performed by the aforementioned controller (not shown). In the following description, the term "high pressure" means a high pressure in the refrigeration 25 cycle (specifically, the pressure at points D, D', and E in FIGS. 3 and 4, and the pressure at points D, D', and F in FIGS. 6 and 7), the term "low pressure" means a low pressure in the refrigeration cycle (specifically, the pressure at points A and F in FIGS. 3 and 4, and the pressure at points A, E, and V in FIGS. 6 and 7), and the term "intermediate pressure" means an intermediate pressure in the refrigeration cycle (specifically, the pressure at points B I and 30 C1 in FIGS. 3 and 4; and the pressure at points B1, C1, and Cl' in FIGS. 6 and 7). <Air-cooling operation> During the air-cooling operation, the switching mechanism 3 is brought to the cooling operation state shown by the solid lines in FIGS. I and 2. The opening degrees of the first expansion mechanism 5a and the second expansion mechanism 5b are adjusted. 16 Since the switching mechanism 3 is set for the cooling operation state, the intermediate heat exchanger on/off valve 12 of the intermediate refrigerant tube 8 is opened and the intermediate heat exchanger bypass on/off valve I1 of the intermediate heat exchanger bypass tube 9 is closed, whereby the intermediate heat exchanger 7 is set to function as a cooler. 5 Additionally, the second intake return on/off valve 92a of the second intake return tube 92 is closed, thereby creating a state in which the intermediate heat exchanger 7 and the intake side of the compression mechanism 2 are not connected (except when air-cooling start control is in effect, described hereinafter), and the intermediate heat exchanger return on/off valve 94a of the intermediate heat exchanger return tube 94 is closed, thereby creating a state in which 10 the intermediate heat exchanger 7 is not connected with the portion between the usage-side heat exchanger 6 and the heat source-side heat exchanger 4. When the refrigerant circuit 10 is in this state, low-pressure refrigerant (refer to point A in FIGS. 1 through 4) is drawn into the compression mechanism 2 through the intake tube 2a, and after the refrigerant is first compressed to an intermediate pressure by the 15 compression element 2c, the refrigerant is discharged to the intermediate refrigerant tube 8 (refer to point BI in FIGS. I through 4). The intermediate-pressure refrigerant discharged from the first-stage compression element 2c is cooled by heat exchange with water or air as a cooling source in the intermediate heat exchanger 7 (refer to point C1 in FIGS. I to 4). The refrigerant cooled in the intermediate heat exchanger 7 is then drawn into and further 20 compressed in the compression element 2d connected to the second-stage side of the compression element 2c, and the refrigerant is then discharged from the compression mechanism 2 to the discharge tube 2b (refer to point D in FIGS. I through 4). The high pressure refrigerant discharged from the compression mechanism 2 is compressed by the two stage compression action of the compression elements 2c, 2d to a pressure exceeding a 25 critical pressure (i.e., the critical pressure Pep at the critical point CP shown in FIG 3). The high-pressure refrigerant discharged from the compression mechanism 2 flows into the oil separator 41a constituting the oil separation mechanism 41, and the accompanying refrigeration oil is separated. The refrigeration oil separated from the high-pressure refrigerant in the oil separator 41a flows into the oil return tube 41b constituting the oil 30 separation mechanism 41 wherein it is depressurized by the pressure-reducing mechanism 41c provided to the oil return tube 41b, and the oil is then returned to the intake tube 2a of the compression mechanism 2 and drawn back into the compression mechanism 2. Next, having been separated from the refrigeration oil in the oil separation mechanism 41, the high pressure refrigerant is passed through the non-return mechanism 42 and the switching 17 mechanism 3, and is fed to the heat source-side heat exchanger 4 functioning as a refrigerant radiator. The high-pressure refrigerant fed to the heat source-side heat exchanger 4 is cooled in the heat source-side heat exchanger 4 by heat exchange with water or air as a cooling source (refer to point E in FIGS. 2 through 4). The high-pressure refrigerant cooled 5 in the heat source-side heat exchanger 4 then flows into the receiver inlet tube 18a through the inlet non-return valve 17a of the bridge circuit 17, and the refrigerant is depressurized to a nearly saturated pressure by the first expansion mechanism 5a and is temporarily retained inside the receiver 18 (refer to point I in FIGS. 1 and 2). The refrigerant retained in the receiver 18 is fed to the receiver outlet tube 18b and is depressurized by the second expansion 10 mechanism 5b to become a low-pressure gas-liquid two-phase refrigerant, and is then fed through the outlet non-return valve 17c of the bridge circuit 17 to the usage-side heat exchanger 6 functioning as a refrigerant evaporator (refer to point F in FIGS. I to 4). The low-pressure gas-liquid two-phase refrigerant fed to the usage-side heat exchanger 6 is heated by heat exchange with water or air as a heating source, and the refrigerant is evaporated as a 15 result (refer to point A in FIGS. 1 to 4). The low-pressure refrigerant heated in the usage side heat exchanger 6 is then drawn back into the compression mechanism 2 via the switching mechanism 3. In this manner the air-cooling operation is performed. Thus, in the air-conditioning apparatus 1 of the present embodiment, the intermediate heat exchanger 7 is provided to the intermediate refrigerant tube 8 for drawing 20 refrigerant discharged from the compression element 2c into the compression element 2d, and in the air-cooling operation the intermediate heat exchanger on/off valve 12 is opened and the intermediate heat exchanger bypass on/off valve 11 of the intermediate heat exchanger bypass tube 9 is closed, thereby putting the intermediate heat exchanger 7 into a state of functioning as a cooler. Therefore, the refrigerant drawn into the compression element 2d on the 25 second-stage side of the compression element 2c decreases in temperature (refer to points B 1 and C1 in FIG 4) and the refrigerant discharged from the compression element 2d also decreases in temperature (refer to points D and D' in FIG 4), in comparison with cases in which no intermediate heat exchanger 7 is provided (in this case, the refrigeration cycle is performed in the sequence in FIGS. 3 and 4: point A -+ point BI - point D' -+ point E -+ 30 point F). Therefore, in the heat source-side heat exchanger 4 functioning as a radiator of high-pressure refrigerant in this air-conditioning apparatus 1, operating efficiency can be improved over cases in which no intermediate heat exchanger 7 is provided, because the temperature difference between the refrigerant and water or air as the cooling source can be reduced, and heat radiation loss can be reduced by an amount equivalent to the area enclosed 18 by connecting points B1, D', D, and C1 in FIG 4. <Air-warming operation> During the air-warming operation, the switching mechanism 3 is brought to the heating operation state shown by the dashed lines in FIGS. 1 and 5. The opening degrees of 5 the first expansion mechanism 5a and the second expansion mechanism 5b are also adjusted. Since the switching mechanism 3 is set to a heating operation state, the intermediate heat exchanger on/off valve 12 of the intermediate refrigerant tube 8 is closed and the intermediate heat exchanger bypass on/off valve 11 of the intermediate heat exchanger bypass tube 9 is opened, thereby putting the intermediate heat exchanger 7 into a state of not functioning as a 10 cooler. Furthermore, since the switching mechanism 3 is set for the heating operation, the second intake return on/off valve 92a of the second intake return tube 92 is opened, thereby creating a state in which the intermediate heat exchanger 7 and the intake side of the compression mechanism 2 are connected, and the intermediate heat exchanger return on/off valve 94a of the intermediate heat exchanger return tube 94 is also opened, thereby creating a 15 state in which the intermediate heat exchanger 7 is connected with the portion between the usage-side heat exchanger 6 and the heat source-side heat exchanger 4. When the refrigerant circuit 10 is in this state, low-pressure refrigerant (refer to point A in FIG 1 and FIGS. 5 L 7) is drawn into the compression mechanism 2 through the intake tube 2a, and after the refrigerant is first compressed to an intermediate pressure by the 20 compression element 2c, the refrigerant is discharged to the intermediate refrigerant tube 8 (refer to point BI in FIG 1 and FIGS. 5 through 7). The intermediate-pressure refrigerant discharged from the first-stage compression element 2c passes through the intermediate heat exchanger bypass tube 9 (refer to point CI in FIG. 1 and FIGS. 5 through 7) without passing through the intermediate heat exchanger 7 (i.e., without being cooled), unlike in the air 25 cooling operation. The refrigerant is drawn into and further compressed in the compression element 2d connected to the second-stage side of the compression element 2c, and is discharged from the compression mechanism 2 to the discharge tube 2b (refer to point D in FIG I and FIGS. 5 through 7). The high-pressure refrigerant discharged from the compression mechanism 2 is compressed by the two-stage compression action of the 30 compression elements 2c, 2d to a pressure exceeding a critical pressure (i.e., the critical pressure Pcp at the critical point CP shown in FIG 6), similar to the air-cooling operation. The high-pressure refrigerant discharged from the compression mechanism 2 flows into the oil separator 41a constituting the oil separation mechanism 41, and the accompanying refrigeration oil is separated. The refrigeration oil separated from the high-pressure 19 refrigerant in the oil separator 41a flows into the oil return tube 41b constituting the oil separation mechanism 41 wherein it is depressurized by the pressure-reducing mechanism 41c provided to the oil return tube 41b, and the oil is then returned to the intake tube 2a of the compression mechanism 2 and drawn back into the compression mechanism 2. Next, 5 having been separated from the refrigeration oil in the oil separation mechanism 41, the high pressure refrigerant is passed through the non-return mechanism 42 and the switching mechanism 3, is fed to the usage-side heat exchanger 6 functioning as a refrigerant radiator, and is cooled by heat exchange with water or air as a cooling source (refer to point F in FIGS. I and FIGS. 5 to 7). The high-pressure refrigerant cooled in the usage-side heat exchanger 6 10 then flows into the receiver inlet tube 18a through the inlet non-return valve 17b of the bridge circuit 17, and the refrigerant is depressurized to a nearly saturated pressure by the first expansion mechanism 5a and is temporarily retained inside the receiver 18 (refer to point I in FIGS. I and 5). The refrigerant retained inside the receiver 18 is fed to the receiver outlet tube 18b and is depressurized by the second expansion mechanism 5b into a low-pressure 15 gas-liquid two-phase refrigerant, which is then fed through the outlet non-return valve 17d of the bridge circuit 17 to the heat source-side heat exchanger 4 functioning as an evaporator of refrigerant, and is also fed through the intermediate heat exchanger return tube 94 to the intermediate heat exchanger 7 functioning as an evaporator of refrigerant (refer to point E in FIGS. I and Figs. 5 to 7). The low-pressure gas-liquid two-phase refrigerant fed to the heat 20 source-side heat exchanger 4 is heated by heat exchange with water or air as a heating source, and the refrigerant evaporates as a result (refer to point A in FIGS. 1, 5 through 7). The low pressure gas-liquid two-phase refrigerant fed to the intermediate heat exchanger 7 is also heated by heat exchange with water or air as a heating source, and the refrigerant evaporates as a result (refer to point V in FIGS. 1, 5 through 7). The low-pressure refrigerant heated 25 and evaporated in the heat source-side heat exchanger 4 is then drawn back into the compression mechanism 2 via the switching mechanism 3. The low-pressure refrigerant heated and evaporated in the intermediate heat exchanger 7 is then drawn back into the compression mechanism 2 via the second intake return tube 92. In this manner the air warming operation is performed. 30 Thus, in the air-conditioning apparatus 1 of the present embodiment, during the air warming operation in which the switching mechanism 3 is set to the heating operation state, the intermediate heat exchanger on/off valve 12 is closed and the intermediate heat exchanger bypass on/off valve 11 is opened, thereby putting the intermediate heat exchanger 7 into a state of not functioning as a cooler. Therefore, the temperature decrease is minimized in the 20 refrigerant discharged from the compression mechanism 2 (refer to points D and D' in FIG 7), in comparison with cases in which only the intermediate heat exchanger 7 is provided and/or cases in which the intermediate heat exchanger 7 is made to function as a cooler similar to the air-cooling operation described above (in these cases, the refrigeration cycle is 5 performed in the sequence in FIGS. 6 and 7: point A -+ point BI -* point Cl' - point D' -+ point F -> point E). Therefore, in the air-conditioning apparatus 1, heat radiation to the exterior can be minimized, temperature decreases can be minimized in the refrigerant supplied to the usage-side heat exchanger 6 functioning as a refrigerant radiator, loss of heating performance can be minimized in proportion to the difference between the enthalpy 10 difference of points D and F and the enthalpy difference of points D' and F in FIG 7, and loss of operating efficiency can be prevented, in comparison with cases in which only the intermediate heat exchanger 7 is provided and/or cases in which the intermediate heat exchanger 7 is made to function as a cooler similar to the air-cooling operation described above. 15 Moreover, in the air-conditioning apparatus 1 of the present embodiment, during the air-warming operation in which the switching mechanism 3 is set for the heating operation state, the intermediate heat exchanger 7 is not merely set to a state of not functioning as a cooler due to not being used. Instead, the intermediate heat exchanger 7, along with the heat source-side heat exchanger 4, is made to function as an evaporator of the refrigerant whose 20 heat has been radiated in the usage-side heat exchanger 6, and is used during the air-warming operation as well to increase refrigerant-evaporating capacity during the air-warming operation and to increase the quantity of refrigerant circulating within the refrigerant circuit 10 while minimizing the heat radiated from the intermediate heat exchanger 7 to the exterior, for example, thereby minimizing the decrease in heating capacity in the usage-side heat 25 exchanger 6. Thereby, in the air-conditioning apparatus I of the present embodiment, heat radiation loss decreases in the heat source-side heat exchanger 4 functioning as a refrigerant radiator during the air-cooling operation, and operating efficiency during the air-cooling operation can be improved. During the air-warming operation, it is possible to use the intermediate heat exchanger 7 more effectively, minimize the reduction in the heating 30 capacity of the heat source-side heat exchanger 6, and prevent operating efficiency during the air-warming operation from decreasing. <Air-cooling start control> With the intermediate heat exchanger 7 described above, there is a risk of liquid refrigerant accumulating when the air-conditioning apparatus I has stopped, or in other 21 instances. When the above-described air-cooling operation is started during a state in which liquid refrigerant has accumulated in the intermediate heat exchanger 7, the liquid refrigerant accumulated in the intermediate heat exchanger 7 is drawn into the second-stage compression element 2d; therefore, the liquid begins to be compressed in the second-stage compression 5 element 2d, and the reliability of the compression mechanism 2 is compromised. In view whereof, in the present embodiment, during the start of the above-described air-cooling operation, a state is created in which the refrigerant discharged from the first stage compression element 2c is drawn into the second-stage compression element 2d through the intermediate heat exchanger bypass tube 9, and air-cooling start control is 10 performed in which the intermediate heat exchanger 7 and the intake side of the compression mechanism 2 are connected by the second intake return tube 92. The air-cooling start control of the present embodiment is described in detail hereinbelow using FIGS. 8 and 9. First, in step S1, when a command to start the air-cooling operation is issued, the 15 process advances to operating the various valves in step S2. Next, in step S2, the on/off state of the on/off valves 11, 12, 92a is switched to a refrigerant return state in which the refrigerant discharged from the first-stage compression element 2c through the intermediate heat exchanger bypass tube 9 is drawn into the second stage compression element 2d, and the intermediate heat exchanger 7 and the intake side of 20 the compression mechanism 2 are connected through the second intake return tube 92. Specifically, the intermediate heat exchanger bypass on/off valve 11 is opened and the intermediate heat exchanger on/off valve 12 is closed. A flow is thus created by the intermediate heat exchanger bypass tube 9 in which the refrigerant discharged from the first stage compression element 2c is drawn into the second-stage compression element 2d without 25 passing through the intermediate heat exchanger 7. In other words, the intermediate heat exchanger 7 is put into a state of not functioning as a cooler, and a state is created in which the refrigerant discharged from the first-stage compression element 2c is drawn into the second-stage compression element 2d through the intermediate heat exchanger bypass tube 9 (see FIG. 9). In this type of state, the second intake return on/off valve 92a is opened. The 30 intermediate heat exchanger 7 and the intake side of the compression mechanism 2 are then connected by the second intake return tube 92, the pressure of the refrigerant in the intermediate heat exchanger 7 (more specifically, in the portion between the intermediate heat exchanger on/off valve 12 and the non-return mechanism 15 which includes the intermediate heat exchanger 7) decreases nearly to the low pressure of the refrigeration cycle, and a state is 22 created in which the refrigerant inside the intermediate heat exchanger 7 can be withdrawn into the intake side of the compression mechanism 2 (see FIG 9). Next, in step S3, the on/off state of the on/off valves 11, 12, 92a in step S2 (i.e., the refrigerant return state) is maintained for a predetermined time duration. Thereby, even if 5 liquid refrigerant has accumulated in the intermediate heat exchanger 7 while the air conditioning apparatus I has been stopped, for example, the liquid refrigerant accumulated inside the intermediate heat exchanger 7 is depressurized and evaporated, then removed out of the intermediate heat exchanger 7 (more specifically, to the intake side of the compression mechanism 2) without being drawn into the second-stage compression element 2d, and is 10 drawn into the compression mechanism 2 (the first-stage compression element 2c, in this case). The predetermined time duration is herein set to a time duration whereby the liquid refrigerant accumulating in the intermediate heat exchanger 7 can be drawn out of the intermediate heat exchanger 7. Next, in step S4, the on/off state of the on/off valves 11, 12, 92a is switched to a 15 refrigerant non-return state in which the refrigerant discharged from the first-stage compression element 2c is drawn into the second-stage compression element 2d through the intermediate heat exchanger 7, and the intermediate heat exchanger 7 and the intake side of the compression mechanism 2 are not connected through the second intake return tube 92. In other words, a transition is made to the on/off state of the valves 11, 12, 92a during the 20 above-described air-cooling operation, and the air-cooling start control is ended. Specifically, the second intake return on/off valve 92a is closed. A state is thus created in which the refrigerant inside the intermediate heat exchanger 7 does not flow out to the intake side of the compression mechanism 2. In this type of state, the intermediate heat exchanger on/off valve 12 is then opened, and the intermediate heat exchanger bypass on/off valve 11 is 25 closed. A state is thus created in which the intermediate heat exchanger 7 functions as a cooler. Thereby, in the air-conditioning apparatus 1, during the start of the air-cooling operation, there is no liquid compression in the second-stage compression element 2d, which would occur as a result of liquid refrigerant accumulating in the intermediate heat exchanger 30 7, and the reliability of the compression mechanism 2 can be improved. (3) Modification I In the embodiment described above, switching between the air-cooling operation and air-cooling start control, i.e., switching between the refrigerant non-return state and the refrigerant return state was performed by changing the on/off state of the on/off valves 11, 12, 23 92a, but another option is a refrigerant circuit 110 as shown in FIG 10, wherein an intermediate heat exchanger switching valve 93 capable of switching between the refrigerant non-return state and the refrigerant return state is provided instead of the on/off valves 11, 12, 92a. 5 The intermediate heat exchanger switching valve 93 herein is a valve capable of switching between the refrigerant non-return state and the refrigerant return state, and in the present modification is a four-way switching valve connected to the discharge side of the first-stage compression element 2c of the intermediate refrigerant tube 8, the inlet side of the intermediate heat exchanger 7 of the intermediate refrigerant tube 8, the end of the 10 intermediate heat exchanger bypass tube 9 on the side near the first-stage compression element 2c, and the end of the second intake return tube 92 on the side near the intermediate heat exchanger 7. The intermediate heat exchanger bypass tube 9 is also provided with a non-return mechanism 9a for allowing refrigerant to flow from the discharge side of the first stage compression element 2c to the intake side of the second-stage compression element 2d 15 and for blocking the refrigerant from flowing from the intake side of the second-stage compression element 2d to the discharge side of the first-stage compression element 2c and the intake side of the compression mechanism 2. The non-return mechanism 9a is a non return valve in the present modification. In the present modification, although a detailed description is not given, the same 20 air-cooling operation as in the embodiment described above can be performed by switching the intermediate heat exchanger switching valve 93 to the refrigerant non-return state (refer to the solid lines of the intermediate heat exchanger switching valve 93 in FIG 10) in which the refrigerant discharged from the first-stage compression element 2c is drawn into the second stage compression element 2d through the intermediate heat exchanger 7 and the intermediate 25 heat exchanger 7 and the intake side of the compression mechanism 2 are not connected through the second intake return tube 92, and the same air-warming operation and/or air cooling start control as in the embodiment described above can be performed by switching the intermediate heat exchanger switching valve 93 to the refrigerant return state (refer to the dashed lines of the intermediate heat exchanger switching valve 93 in FIG 10) in which the 30 refrigerant discharged from the first-stage compression element 2c is drawn into the second stage compression element 2d through the intermediate heat exchanger bypass tube 9 and the intermediate heat exchanger 7 and the intake side of the compression mechanism 2 are connected through the second intake return tube 92. The same operational effects as those of the embodiment described above can also 24 be achieved with the configuration of the present modification. Moreover, in the present modification, since the refrigerant non-return state and the refrigerant return state can be switched by the intermediate heat exchanger switching valve 93, the number of valves can be reduced in comparison with the case of using the configuration in which the refrigerant non 5 return state and the refrigerant return state are switched by a plurality of valves 11, 12, 92a such as those of the embodiment described above. Since pressure loss is reduced in comparison with cases of using electromagnetic valves, the decrease in intermediate pressure in the refrigeration cycle can be minimized, and the decrease operating efficiency can also be minimized. 10 (4) Modification 2 In the above-described embodiment and the modification thereof, consideration is given to using a configuration in which the intermediate heat exchanger 7 and the heat source-side heat exchanger 4 are heat exchangers that use air as a heat source (that is, as a cooling source or a heating source), and air as a heat source is supplied by a common heat 15 source-side fan 40 (described hereinafter) to both heat exchangers 4, 7. For example, in cases in which the air-conditioning apparatus I has a configuration in which a heat source unit la provided primarily with the heat source-side fan 40, the heat source-side heat exchanger 4, and the intermediate heat exchanger 7 is connected with a usage unit (not shown) provided primarily with the usage-side heat exchanger 6, the heat 20 source unit la such as is shown in FIGS. 11 and 12 could possibly be used. Here, FIG 11 is an external perspective view of the heat source unit la (the fan grill having been removed), and FIG 12 is a side view of the heat source unit Ia with the right plate of the heat source unit la having been removed. The terms "left" and "right" in the following description refer to a case of viewing the heat source unit Ia from the side of the front plate. 25 The heat source unit la constituting the air-conditioning apparatus I of the present modification is a so-called upward-blowing type in which air is drawn in from the sides and the air is blown upwards, the heat source unit la having primarily a casing 71, and disposed inside the casing 71 a heat source-side heat exchanger 4, a intermediate heat exchanger 7, and other refrigerant circuit structural components, and/or a heat source-side fan 40 and other 30 devices. In the present modification, the casing 71 is a substantially rectangular parallelepiped-shaped box, and is configured primarily from a top plate 72 constituting the top surface of the casing 71, a left plate 73 constituting the external peripheral surface of the casing 71, a right plate 74, a front plate 75, a rear plate 76, and a bottom plate 77. The top 25 plate 72 is primarily a member constituting the top surface of the casing 71, and in the present modification is a plate-shaped member seen as a substantial rectangle in a plan view, an air blowing opening 71a being formed substantially in the middle. A fan grill 78 is provided to the top plate 72 so as to cover the air-blowing opening 71a from above. The left plate 73 is 5 primarily a member constituting the left surface of the casing 71, and in the present modification is plate-shaped member seen as a substantial rectangle in a side view, extending downward from the left edge of the top plate 72. Intake openings 73a are formed throughout almost the entire left plate 73, except for the top part. The right plate 74 is primarily a member constituting the right surface of the casing 71, and in the present 10 modification is a plate-shaped member seen as a substantial rectangle in a side view, extending downward from the right edge of the top plate 72. Intake openings 74a are formed throughout almost the entire right plate 74, except for the top part. The front plate 75 is a member primarily constituting the front surface of the casing 71, and in the present modification is configured from a plate-shaped member seen as a substantial rectangle in a 15 front view, disposed in sequence below the front edge of the top plate 72. The rear plate 76 is primarily a member constituting the rear surface of the casing 71, and in the present modification is configured from a plate-shaped member seen as a substantial rectangle in a front view, disposed in sequence below the rear edge of the top plate 72. Intake openings 76a are formed throughout almost the entire rear plate 76, except for the top part. The 20 bottom plate 77 is primarily a member constituting the bottom surface of the casing 71, and in the present modification is a plate-shaped member seen as a substantial rectangle in a plan view. In the present modification, the intermediate heat exchanger 7 is integrated with the heat source-side heat exchanger 4 in a state of being disposed above the heat source-side heat 25 exchanger 4, and is also disposed on top of the bottom plate 77. More specifically, the intermediate heat exchanger 7 is integrated with the heat source-side heat exchanger 4 due to sharing heat transfer fins. The integration of the heat source-side heat exchanger 4 and the intermediate heat exchanger 7 forms a heat exchanger panel seen as a substantial U shape in a plan view in the present modification, and this panel is disposed so as to face the intake 30 openings 73a, 74a, 76a. The heat source-side fan 40 is directed towards the air-blowing opening 71 a of the top plate 72, and is disposed on the top side of the integration of the heat source-side heat exchanger 4 and the intermediate heat exchanger 7 (i.e., the heat exchanger panel). In the present modification, the heat source-side fan 40 is an axial flow fan and is rotatably driven by a fan drive motor 40a, whereby air as a heat source is drawn from the 26 intake openings 73a, 74a, 76a into the casing 71, and after passing through the heat source side heat exchanger 4 and the intermediate heat exchanger 7, the air can be blown upwards from the air-blowing opening 71a (refer to the arrows indicating the flow of air in FIG 12). Specifically, the heat source-side fan 40 is designed so as to supply air as a heat source to 5 both the heat source-side heat exchanger 4 and the intermediate heat exchanger 7. The external shape of the heat source unit la and/or the shape of the integration of the heat source-side heat exchanger 4 and intermediate heat exchanger 7 (i.e., the heat exchanger panel) are not limited to those described above. Thus, the intermediate heat exchanger 7 constitutes a heat exchanger panel integrated with the heat source-side heat exchanger 4, and 10 the intermediate heat exchanger 7 is disposed in the top part of the heat exchanger panel. The intermediate heat exchanger 7 and the heat source-side heat exchanger 4 are integrated and the intermediate heat exchanger 7 is disposed in the top part of the heat exchanger panel consisting of the two integrated heat exchangers in light of the fact that the air-conditioning apparatus I of the present modification uses a refrigerant that operates in a 15 supercritical range (carbon dioxide in this case), and also the fact that the heat source unit la is a model that draws air in from the sides and blows the air upward. To describe these facts in detail, sometimes the refrigeration cycle of the air-cooling operation, for example, is performed (see FIG. 3) in which refrigerant of an intermediate pressure lower than the critical pressure Pcp (approximately 7.3 MPa with carbon dioxide) flows into the intermediate heat 20 exchanger 7 as a cooler during the air-cooling operation, and refrigerant of a high pressure exceeding the critical pressure Pcp flows into the heat source-side heat exchanger 4 functioning as a refrigerant radiator. In this case, as a result of the difference between the properties of the refrigerant at a pressure lower than the critical pressure Pcp and the properties (particularly the heat transfer coefficient and the specific heat at constant pressure) 25 of the refrigerant at a pressure exceeding the critical pressure Pep, there is a tendency for the heat transfer coefficient of the refrigerant in the intermediate heat exchanger 7 as a cooler to be lower than the heat transfer coefficient of the refrigerant in the heat source-side heat exchanger 4 functioning as a refrigerant radiator, as shown in FIG 13. Here, FIG 13 shows the heat transfer coefficient value (corresponding to the heat transfer coefficient of the 30 refrigerant in the intermediate heat exchanger 7 as a cooler) in a case in which carbon dioxide at 6.5 MPa flows at a predetermined mass flow rate into a heat transfer passage having a predetermined passage cross-sectional surface area, as well as the heat transfer coefficient value (corresponding to the heat transfer coefficient of the refrigerant in the heat source-side heat exchanger 4 as a radiator) of carbon dioxide at 10 MPa in the same heat transfer passage 27 and under the same mass flow rate conditions as the 6.5 MPa carbon dioxide. It is clear from this diagram that in the temperature range (35 to 70 degrees) of the refrigerant flowing within the heat source-side heat exchanger 4 functioning as a refrigerant radiator and/or the intermediate heat exchanger 7 functioning as a cooler of refrigerant, the heat transfer 5 coefficient value of the 6.5 MPa carbon dioxide will be less than the heat transfer coefficient value of the 10 MPa carbon dioxide. Therefore, in the heat source unit la of the air conditioning apparatus 1 of the present modification (i.e., in a heat source unit configured so as to take air in from the sides and blow air upwards), when the intermediate heat exchanger 7 is tentatively integrated with the heat source-side heat exchanger 4 in a state of being 10 disposed below the heat source-side heat exchanger 4, the intermediate heat exchanger 7 integrated with the heat source-side heat exchanger 4 is disposed in the bottom part of the heat source unit 1 a where the flow rate of air as the heat source is low, and the effects of the reduction in the heat transfer coefficient of the air in the intermediate heat exchanger 7 caused by disposing the intermediate heat exchanger 7 in the bottom part of the heat source unit la 15 combine with the effects of the heat transfer coefficient of the refrigerant in the intermediate heat exchanger 7 being lower than the heat transfer coefficient of the refrigerant in the heat source-side heat exchanger 4, which is because there is a reduction in the heat transfer performance of the intermediate heat exchanger 7. In this type of heat source unit la, if the intermediate heat exchanger bypass tube 9 20 were to be used during the air-warming operation so that the refrigerant discharged from the first-stage compression element 2c and drawn into the second-stage compression element 2d bypasses the intermediate heat exchanger 7 so as not to be cooled therein and the intermediate heat exchanger 7 is not used, there would be a severe disadvantage in that the intermediate heat exchanger 7, which is disposed in a position where the flow rate of air as a 25 heat source is fastest in light of the heat transfer coefficient during the air-cooling operation, would not contribute at all during the air-warming operation, and the intermediate heat exchanger 7 would not be effectively used. However, during the air-warming operation in the present modification, as in the above-described embodiment and the modification thereof, the intermediate heat exchanger 30 bypass tube 9 is used so that the refrigerant discharged from the first-stage compression element 2c and drawn into the second-stage compression element 2d bypasses the intermediate heat exchanger 7 so as not to be cooled therein, and the intermediate heat exchanger 7 is made to function as an evaporator of refrigerant, thereby contributing to improving the evaporation capacity during the air-warming operation. 28 (5) Modification 3 In the above-described embodiment and the modifications thereof, the air conditioning apparatus 1 which is configured to be capable of switching between the air cooling operation and the air-warming operation via the switching mechanism 3 and which 5 performs a two-stage compression refrigeration cycle is provided with an intermediate heat exchanger 7 that functions as a cooler of the refrigerant discharged from the first-stage compression element 2c and drawn into the second-stage compression element 2d, an intermediate heat exchanger bypass tube 9 connected to the intermediate refrigerant tube 8 so as to bypass the intermediate heat exchanger 7, a second intake return tube 92 for connecting 10 one end of the intermediate heat exchanger 7 and the intake side of the compression mechanism 2, and an intermediate heat exchanger return tube 94 for connecting the portion between the usage-side heat exchanger 6 and the heat source-side heat exchanger 4 with the other end of the intermediate heat exchanger 7, but in addition to this configuration, intermediate pressure injection may be performed by a first second-stage injection tube 19 15 and an economizer heat exchanger 20. For example, the refrigerant circuit 10 (see FIG 1) of the above-described embodiment in which the two-stage compression-type compression mechanism 2 is used can be replaced by a refrigerant circuit 210 provided with the first second-stage injection tube 19 and the economizer heat exchanger 20, as shown in FIG 14. 20 The first second-stage injection tube 19 has a function for branching off and returning the refrigerant flowing between the heat source-side heat exchanger 4 and the usage-side heat exchanger 6 to the second-stage compression element 2d of the compression mechanism 2. In the present modification, the first second-stage injection tube 19 is provided so as to branch off refrigerant flowing through the receiver inlet tube 18a and return 25 the refrigerant to the second-stage compression element 2d. More specifically, the first second-stage injection tube 19 is provided so as to branch off refrigerant from a position upstream of the first expansion mechanism 5a of the receiver inlet tube 18a (i.e., a position between the heat source-side heat exchanger 4 and the first expansion mechanism 5a when the switching mechanism 3 is set to the cooling operation state) and return the refrigerant to a 30 position in the intermediate refrigerant tube 8 downstream of the intermediate heat exchanger 7. The first second-stage injection tube 19 is provided with a first second-stage injection valve 19a whose opening degree can be controlled. The first second-stage injection valve 19a is an electrically driven expansion valve in the present modification. The economizer heat exchanger 20 is a heat exchanger for carrying out heat 29 exchange between the refrigerant flowing between the heat source-side heat exchanger 4 and the usage-side heat exchanger 6 and the refrigerant that flows through the first second stage injection tube 19 (more specifically, the refrigerant that has been depressurized to a nearly intermediate pressure in the first second-stage injection valve 19a). In the present 5 modification, the economizer heat exchanger 20 is provided so as to perform heat exchange between the refrigerant flowing through a position in the receiver inlet tube 18a upstream of the first expansion mechanism 5a (i.e., between the heat source-side heat exchanger 4 and the first expansion mechanism 5a when the switching mechanism 3 is set to the cooling operation state) and the refrigerant flowing through the first second-stage injection tube 19, and the 10 economizer heat exchanger 20 has a flow passage through which both refrigerants flow against each other. In the present modification, the economizer heat exchanger 20 is provided farther downstream than the position where the first second-stage injection tube 19 branches from the receiver inlet tube 18a. Therefore, the refrigerant flowing between the heat source-side heat exchanger 4 and the usage-side heat exchanger 6 is branched off in the 15 receiver inlet tube 18a into the first second-stage injection tube 19 before undergoing heat exchange in the economizer heat exchanger 20, and heat exchange is then conducted in the economizer heat exchanger 20 with the refrigerant flowing through the first second-stage injection tube 19. Thus, in the present modification, when the switching mechanism 3 is set to the 20 cooling operation state, the high-pressure refrigerant cooled in the heat source-side heat exchanger 4 can be fed to the usage-side heat exchanger 6 through the inlet non-return valve 17a of the bridge circuit 17, the economizer heat exchanger 20, the first expansion mechanism 5a of the receiver inlet tube 18a, the receiver 18, the second expansion mechanism 5b of the receiver outlet tube 18b, and the outlet non-return valve 17c of the 25 bridge circuit 17. When the switching mechanism 3 is brought to the heating operation state, the high-pressure refrigerant cooled in the usage-side heat exchanger 6 can be fed to the heat source-side heat exchanger 4 through the inlet non-return valve 17b of the bridge circuit 17, the economizer heat exchanger 20, the first expansion mechanism 5a of the receiver inlet tube 18a, the receiver 18, the second expansion mechanism 5b of the receiver outlet tube 18b, 30 and the outlet non-return valve 17d of the bridge circuit 17. Furthermore, in the present modification, the intermediate refrigerant tube 8 or the compression mechanism 2 is provided with an intermediate pressure sensor 54 for detecting the pressure of the refrigerant that flows through the intermediate refrigerant tube 8. The outlet of the first second-stage injection tube 19 side of the economizer heat exchanger 20 is 30 provided with an economizer outlet temperature sensor 55 for detecting the temperature of the refrigerant at the outlet of the first second-stage injection tube 19 side of the economizer heat exchanger 20. Next, the action of the air-conditioning apparatus I of the present modification will 5 be described using FIGS. 14 through 18. FIG 15 is a pressure-enthalpy graph representing the refrigeration cycle during the air-cooling operation, FIG. 16 is a temperature-entropy graph representing the refrigeration cycle during the air-cooling operation, FIG 17 is a pressure-enthalpy graph representing the refrigeration cycle during the air-warming operation, and FIG 18 is a temperature-entropy graph representing the refrigeration cycle 10 during the air-warming operation. This air-cooling start control is the same as that of the embodiment described above and is therefore not described herein. Operation control in the air-cooling operation and air-warming operation described below (including the air-cooling start control not described herein) is performed by the controller (not shown) in the embodiment described above. In the following description, the term "high pressure" means 15 a high pressure in the refrigeration cycle (specifically, the pressure at points D, D', E, and H in FIGS. 15 and 16, and the pressure at points D, D', F, and H in FIGS. 17 and 18), the term "low pressure" means a low pressure in the refrigeration cycle (specifically, the pressure at points A and F in FIGS. 15 and 16, and the pressure at points A, E, V in FIGS. 17 and 18), and the term "intermediate pressure" means an intermediate pressure in the refrigeration cycle 20 (specifically, the pressure at points BI, C1, G, J, and K in FIGS. 15 through 18). <Air-cooling operation> During the air-cooling operation, the switching mechanism 3 is brought to the cooling operation state shown by the solid lines in FIG 14. The opening degrees of the first expansion mechanism 5a and the second expansion mechanism 5b are adjusted. 25 Furthermore, the opening degree of the first second-stage injection valve 19a is also adjusted. More specifically, in the present modification, what is known as superheat degree control is performed, wherein the opening degree of the first second-stage injection valve 19a is adjusted so that a target value is achieved in the degree of superheat of the refrigerant at the outlet in the first second-stage injection tube 19 side of the economizer heat exchanger 20. 30 In the present modification, the degree of superheat of the refrigerant at the outlet in the first second-stage injection tube 19 side of the economizer heat exchanger 20 is obtained by converting the intermediate pressure detected by the intermediate pressure sensor 54 to a saturation temperature and subtracting this refrigerant saturation temperature value from the refrigerant temperature detected by the economizer outlet temperature sensor 55. Though 31 not used in the present modification, another possible option is to provide a temperature sensor to the inlet in the first second-stage injection tube 19 side of the economizer heat exchanger 20, and to obtain the degree of superheat of the refrigerant at the outlet in the first second-stage injection tube 19 side of the economizer heat exchanger 20 by subtracting the 5 refrigerant temperature detected by this temperature sensor from the refrigerant temperature detected by the economizer outlet temperature sensor 55. Adjusting the opening degree of the first second-stage injection valve 19a is not limited to the superheat degree control, and the valve may be opened to predetermined opening degree in accordance with the flow rate of refrigerant circulating in the refrigerant circuit 10, for example. Since the switching 10 mechanism 3 is set to the cooling operation state, a state in which the intermediate heat exchanger 7 functions as a cooler is created by opening the intermediate heat exchanger on/off valve 12 of the intermediate refrigerant tube 8 and closing the intermediate heat exchanger bypass on/off valve 11 of the intermediate heat exchanger bypass tube 9, a state in which the intermediate heat exchanger 7 and the intake side of the compression mechanism 2 15 are not connected is created (except during air-cooling start control) by closing the second intake return on/off valve 92a of the second intake return tube 92, and a state in which the portion between the usage-side heat exchanger 6 and the heat source-side heat exchanger 4 is not connected with the intermediate heat exchanger 7 is created by closing the intermediate heat exchanger return on/off valve 94a of the intermediate heat exchanger return tube 94. 20 When the refrigerant circuit 210 is in this state, low-pressure refrigerant (refer to point A in FIGS. 14 through 16) is drawn into the compression mechanism 2 through the intake tube 2a, and after the refrigerant is first compressed to an intermediate pressure by the compression element 2c, the refrigerant is discharged to the intermediate refrigerant tube 8 (refer to point BI in FIGS. 14 through 16). The intermediate-pressure refrigerant 25 discharged from the first-stage compression element 2c is cooled by heat exchange with water or air as a cooling source in the intermediate heat exchanger 7 (refer to point C1 in FIGS. 14 to 16). The refrigerant cooled in the intermediate heat exchanger 7 is further cooled (refer to point G in FIGS. 14 to 16) by being mixed with refrigerant being returned from the first second-stage injection tube 19 to the second-stage compression element 2d 30 (refer to point K in FIGS. 14 to 16). Next, having been mixed with the refrigerant returning from the first second-stage injection tube 19 (i.e., intermediate pressure injection is carried out by the economizer heat exchanger 20), the intermediate-pressure refrigerant is drawn into and further compressed in the compression element 2d connected to the second-stage side of the compression element 2c, and the refrigerant is discharged from the compression 32 mechanism 2 to the discharge tube 2b (refer to point D in FIGS. 14 through 16). The high pressure refrigerant discharged from the compression mechanism 2 is compressed by the two stage compression action of the compression elements 2c, 2d to a pressure exceeding a critical pressure (i.e., the critical pressure Pep at the critical point CP shown in FIG. 15). 5 The high-pressure refrigerant discharged from the compression mechanism 2 flows into the oil separator 41a constituting the oil separation mechanism 41, and the accompanying refrigeration oil is separated. The refrigeration oil separated from the high-pressure refrigerant in the oil separator 41a flows into the oil return tube 41b constituting the oil separation mechanism 41 wherein it is depressurized by the pressure-reducing mechanism 10 41c provided to the oil return tube 41b, and the oil is then returned to the intake tube 2a of the compression mechanism 2 and drawn back into the compression mechanism 2. Next, having been separated from the refrigeration oil in the oil separation mechanism 41, the high pressure refrigerant is passed through the non-return mechanism 42 and the switching mechanism 3, and is fed to the heat source-side heat exchanger 4 functioning as a refrigerant 15 radiator. The high-pressure refrigerant fed to the heat source-side heat exchanger 4 is cooled in the heat source-side heat exchanger 4 by heat exchange with water or air as a cooling source (refer to point E in FIGS. 14 through 16). The high-pressure refrigerant cooled in the heat source-side heat exchanger 4 flows through the inlet non-return valve 17a of the bridge circuit 17 into the receiver inlet tube 18a, and some of the refrigerant is 20 branched off into the first second-stage injection tube 19. The refrigerant flowing through the first second-stage injection tube 19 is depressurized to a nearly intermediate pressure in the first second-stage injection valve 19a and is then fed to the economizer heat exchanger 20 (refer to point J in FIGS. 14 to 16). The refrigerant branched off to the first second-stage injection tube 19 then flows into the economizer heat exchanger 20, where it is cooled by 25 heat exchange with the refrigerant flowing through the first second-stage injection tube 19 (refer to point H in FIGS. 14 to 16). The refrigerant flowing through the first second-stage injection tube 19 is heated by heat exchange with the high-pressure refrigerant cooled in the heat source-side heat exchanger 4 as a radiator (refer to point K in FIGS. 14 through 16), and is then mixed with the intermediate-pressure refrigerant discharged from the first-stage 30 compression element 2c as described above. The high-pressure refrigerant cooled in the economizer heat exchanger 20 is depressurized to a nearly saturated pressure by the first expansion mechanism 5a and is temporarily retained in the receiver 18 (refer to point I in FIG. 14). The refrigerant retained in the receiver 18 is fed to the receiver outlet tube 18b and is depressurized by the second expansion mechanism 5b to become a low-pressure gas 33 liquid two-phase refrigerant, and is then fed through the outlet non-return valve 17c of the bridge circuit 17 to the usage-side heat exchanger 6 functioning as a refrigerant evaporator (refer to point F in FIGS. 14 to 16). The low-pressure gas-liquid two-phase refrigerant fed to the usage-side heat exchanger 6 is heated by heat exchange with water or air as a heating 5 source, and the refrigerant is evaporated as a result (refer to point A in FIGS. 14 to 16). The low-pressure refrigerant heated in the usage-side heat exchanger 6 is then drawn back into the compression mechanism 2 via the switching mechanism 3. In this manner the air-cooling operation is performed. In the configuration of the present modification, as in Modification 2 described 10 above, since the intermediate heat exchanger 7 is in a state of functioning as a cooler during the air-cooling operation in which the switching mechanism 3 is brought to the cooling operation state, heat radiation loss in the heat source-side heat exchanger 4 can be reduced in comparison with cases in which no intermediate heat exchanger 7 is provided. Moreover, in the configuration of the present modification, since the first second 15 stage injection tube 19 and the economizer heat exchanger 20 are provided so as to branch off refrigerant fed from the heat source-side heat exchanger 4 to the expansion mechanisms 5a, 5b and return the refrigerant to the second-stage compression element 2d, the temperature of refrigerant drawn into the second-stage compression element 2d can be kept even lower (refer to points Cl and G in FIG. 16) without performing heat radiation to the exterior, such as is 20 done with the intermediate heat exchanger 7. The temperature of the refrigerant discharged from the compression mechanism 2 is thereby kept even lower (refer to points D and D' in FIG 16), and operating efficiency can be further improved because heat radiation loss can be further reduced in proportion to the area enclosed by connecting the points Cl, D', D, and G in FIG 16, in comparison with cases in which no first second-stage injection tube 19 is 25 provided. In the present modification, as in Modification 2 described above, at the start of the air-cooling operation in which the switching mechanism 3 is set to the cooling operation state, the refrigerant discharged from the first-stage compression element 2c is drawn into the second-stage compression element 2d through the intermediate heat exchanger bypass tube 9, 30 and the intermediate heat exchanger 7 and the intake side of the compression mechanism 2 are connected through the second intake return tube 92. Therefore, even if liquid refrigerant has accumulated in the intermediate heat exchanger 7 prior to the start of the operation in which the switching mechanism 3 is set to the cooling operation state, the liquid refrigerant can be removed from the intermediate heat exchanger 7. Thereby, at the start of the 34 operation in which the switching mechanism 3 is set to the cooling operation state, it is possible to avoid a state of liquid refrigerant accumulating inside the intermediate heat exchanger 7, liquid compression does not occur in the second-stage compression element 2d as a result of liquid refrigerant accumulating in the intermediate heat exchanger 7, and the 5 reliability of the compression mechanism 2 can be improved. <Air-warming operation> During the air-warming operation, the switching mechanism 3 is brought to the heating operation state shown by the dashed lines in FIG. 14. The opening degrees of the first expansion mechanism 5a and the second expansion mechanism 5b are adjusted. The 10 first second-stage injection valve 19a is also subjected to the same opening degree adjustment as in the air-cooling operation described above. Since the switching mechanism 3 is set to the heating operation state, the intermediate heat exchanger on/off valve 12 of the intermediate refrigerant tube 8 is closed and the intermediate heat exchanger bypass on/off valve 11 of the intermediate heat exchanger bypass tube 9 is opened, thereby putting the 15 intermediate heat exchanger 7 into a state of not functioning as a cooler. Furthermore, since the switching mechanism 3 is in the heating operation state, a state in which the intermediate heat exchanger 7 and the intake side of the compression mechanism 2 are connected is created by opening the second intake return on/off valve 92a of the second intake return tube 92, and a state in which the portion between the usage-side heat exchanger 6 and the heat 20 source-side heat exchanger 4 is connected with the intermediate heat exchanger 7 is created by opening the intermediate heat exchanger return on/off valve 94a of the intermediate heat exchanger return tube 94. When the refrigerant circuit 210 is in this state, low-pressure refrigerant (refer to point A in FIGS. 14, 17, and 18) is drawn into the compression mechanism 2 through the 25 intake tube 2a, and after the refrigerant is first compressed to an intermediate pressure by the compression element 2c, the refrigerant is discharged to the intermediate refrigerant tube 8 (refer to point BI in FIGS. 14, 17, 18). Unlike in the air-cooling operation, the intermediate-pressure refrigerant discharged from the first-stage compression element 2c passes through the intermediate heat exchanger bypass tube 9 (refer to point Cl in FIGS. 14, 30 17, and 18) without passing through the intermediate heat exchanger 7 (i.e., without being cooled), and the refrigerant is cooled (refer to point G in FIGS. 14, 17, and 18) by being mixed with refrigerant being returned from the first second-stage injection tube 19 to the second-stage compression element 2d (refer to point K in FIGS. 14, 17, and 18). Next, having been mixed with the refrigerant returning from the first second-stage injection tube 35 19, the intermediate-pressure refrigerant is drawn into and further compressed in the compression element 2d connected to the second-stage side of the compression element 2c, and the refrigerant is discharged from the compression mechanism 2 to the discharge tube 2b (refer to point D in FIGS. 14, 17, and 18). The high-pressure refrigerant discharged from 5 the compression mechanism 2 is compressed by the two-stage compression action of the compression elements 2c, 2d to a pressure exceeding a critical pressure (i.e., the critical pressure Pcp at the critical point CP shown in FIG 17), similar to the air-cooling operation. The high-pressure refrigerant discharged from the compression mechanism 2 flows into the oil separator 41a constituting the oil separation mechanism 41, and the accompanying 10 refrigeration oil is separated. The refrigeration oil separated from the high-pressure refrigerant in the oil separator 41a flows into the oil return tube 41b constituting the oil separation mechanism 41 wherein it is depressurized by the pressure-reducing mechanism 41c provided to the oil return tube 41b, and the oil is then returned to the intake tube 2a of the compression mechanism 2 and drawn back into the compression mechanism 2. Next, after 15 the refrigeration oil has been separated in the oil separation mechanism 41, the high-pressure refrigerant is fed through the non-return mechanism 42 and the switching mechanism 3 to the usage-side heat exchanger 6 functioning as a refrigerant radiator, and is cooled by heat exchange with water and/or air as a cooling source (refer to point F in FIGS. 14, 17, and 18). The high-pressure refrigerant cooled in the usage-side heat exchanger 6 flows through the 20 inlet non-return valve 17b of the bridge circuit 17 into the receiver inlet tube 18a, and some of the refrigerant is branched off into the first second-stage injection tube 19. The refrigerant flowing through the first second-stage injection tube 19 is depressurized to a nearly intermediate pressure in the first second-stage injection valve 19a and is then fed to the economizer heat exchanger 20 (refer to point J in FIGS. 14, 17, and 18). The refrigerant 25 branched off to the first second-stage injection tube 19 then flows into the economizer heat exchanger 20, where it is cooled by heat exchange with the refrigerant flowing through the first second-stage injection tube 19 (refer to point H in FIGS. 14, 17, and 18). The refrigerant flowing through the first second-stage injection tube 19 is heated by heat exchange with the high-pressure refrigerant cooled in the heat source-side heat exchanger 4 30 as a radiator (refer to point K in FIGS. 14, 17, and 18), and is mixed with the intermediate pressure refrigerant discharged from the first-stage compression element 2c as described above. The high-pressure refrigerant cooled in the economizer heat exchanger 20 is depressurized to a nearly saturated pressure by the first expansion mechanism 5a and is temporarily retained in the receiver 18 (refer to point I in FIG. 14). The refrigerant retained 36 in the receiver 18 is then fed to the receiver outlet tube 18b and depressurized by the second expansion mechanism 5b to become low-pressure gas-liquid two-phase refrigerant, which is then fed through the outlet non-return valve 17d of the bridge circuit 17 to the heat source side heat exchanger 4 functioning as an evaporator of refrigerant, and is also fed through the 5 intermediate heat exchanger return tube 94 to the intermediate heat exchanger 7 functioning as an evaporator of refrigerant (refer to point E in FIGS. 14, 17, and 18). The low-pressure gas-liquid two-phase refrigerant fed to the heat source-side heat exchanger 4 is heated by heat exchange with water or air as a heating source, and the refrigerant evaporates as a result (refer to point A in FIGS. 14, 17, and 18). The low-pressure gas-liquid two-phase 10 refrigerant fed to the intermediate heat exchanger 7 is also heated by heat exchange with water or air as a heating source, and the refrigerant evaporates as a result (refer to point V in FIGS. 14, 17, and 18). The low-pressure refrigerant heated and evaporated in the heat source-side heat exchanger 4 is then drawn back into the compression mechanism 2 via the switching mechanism 3. The low-pressure refrigerant heated and evaporated in the 15 intermediate heat exchanger 7 is then drawn back into the compression mechanism 2 via the second intake return tube 92. In this manner the air-warming operation is performed. In the configuration of the present modification, as in Modification 2 described above, during the air-warming operation in which the switching mechanism 3 is set to the heating operation state, heat radiation to the exterior can be minimized, the decrease of 20 heating capacity can be minimized, and decreases in operating efficiency can be prevented in comparison with cases in which only the intermediate heat exchanger 7 is provided and/or cases in which the intermediate heat exchanger 7 is made to function as a cooler, similar to the air-cooling operation described above. Moreover, in the configuration of the present modification, since the first second 25 stage injection tube 19 and the economizer heat exchanger 20 are provided so as to branch off refrigerant fed from the heat source-side heat exchanger 4 to the expansion mechanisms 5a, 5b and return the refrigerant to the second-stage compression element 2d in the same manner as the air-cooling operation, the temperature of refrigerant drawn into the second-stage compression element 2d can be kept even lower (refer to points BI and G in FIG. 18) without 30 performing heat radiation to the exterior, such as is done with the intermediate heat exchanger 7. The temperature of refrigerant discharged from the compression mechanism 2 is thereby kept even lower (refer to points D and D' in FIG. 18), and operating efficiency can be further improved because heat radiation loss can be reduced in proportion to the area enclosed by connecting the points B 1, D', D, and G in FIG. 18, in comparison with cases in which no first 37 second-stage injection tube 19 is provided. In the configuration of the present modification, similar to the embodiment described above, during the air-cooling operation, there is less heat radiation loss in the heat source-side heat exchanger 4 functioning as a refrigerant radiator, and the operating 5 efficiency during the air-cooling operation can be improved. During the air-warming operation, it is possible to effectively use the intermediate heat exchanger 7, minimize the loss of heating capacity in the usage-side heat exchanger 6, and prevent the operating efficiency during the air-warming operation from decreasing. Advantages of both the air-cooling operation and the air-warming operation in the 10 configuration of the present modification are that the economizer heat exchanger 20 is a heat exchanger which has flow channels through which refrigerant fed from the heat source-side heat exchanger 4 or usage-side heat exchanger 6 to the expansion mechanisms Sa, Sb and refrigerant flowing through the second-stage injection tube 19 both flow so as to oppose each other; therefore, it is possible to reduce the temperature difference between the refrigerant fed 15 to the expansion mechanisms 5a, 5b from the heat source-side heat exchanger 4 or the usage side heat exchanger 6 in the economizer heat exchanger 20 and the refrigerant flowing through the second-stage injection tube 19, and high heat exchange efficiency can be obtained. In the present modification, switching between the air-cooling operation and the air 20 cooling start control, i.e., switching between the refrigerant non-return state and the refrigerant return state is performed through the on/off states of the on/off valves 11, 12, 92a, but another option instead of the on/off valves 11, 12, 92a is to provide an intermediate heat exchanger switching valve 93 capable of switching between the refrigerant non-return state and the refrigerant return state, as in Modification 1 described above. 25 Furthermore, particularly advantageous effects can be obtained when using the configuration of the heat source unit la, such as the one in Modification 2. (6) Modification 4 In the refrigerant circuit 210 (see FIG 14) in Modification 3 described above, in both the air-cooling operation in which the switching mechanism 3 is set to the cooling operation 30 state and the air-warming operation in which the switching mechanism 3 is set to the heating operation state as described above, performing intermediate pressure injection through the economizer heat exchanger 20 reduces the temperature of the refrigerant discharged from the second-stage compression element 2d, reduces the power consumption of the compression mechanism 2, and makes it possible to improve operating efficiency. Intermediate pressure 38 injection by the economizer heat exchanger 20 can be used in conditions in which the intermediate pressure in the refrigeration cycle has increased to a nearly critical pressure, which is believed to be particularly advantageous in cases in which refrigerant that operates in a supercritical range is used with a configuration having a single usage-side heat exchanger 5 6, such as the refrigerant circuits 10, 110, 210 (see FIGS. 1, 10, and 14) in the above described embodiment and the modifications thereof. However, in order to create a configuration having a plurality of usage-side heat exchangers 6 connected in parallel to each other, and to make it possible to control the flow rate of refrigerant flowing through the usage-side heat exchangers 6 and obtain the 10 refrigeration load required by the usage-side heat exchangers 6, the objective being to perform air-cooling or air-warming corresponding to the air-conditioning loads of a plurality of air-conditioned spaces, for example; in some cases usage-side expansion mechanisms 5c are provided between the receiver 18 as a gas-liquid separator and the usage-side heat exchangers 6 so as to correspond to the usage-side heat exchangers 6. 15 For example, although the details are not shown, one possibility in the refrigerant circuit 210 (see FIG 14) having the bridge circuit 17 in Modification 3 described above is that a plurality (two herein) of usage-side heat exchangers 6 connected to each other in parallel be provided, usage-side expansion mechanisms 5c (see FIG 19) be provided between the receiver 18 as a gas-liquid separator (more specifically, the bridge circuit 17) and the 20 usage-side heat exchangers 6 so as to correspond to the usage-side heat exchangers 6, the second expansion mechanism 5b that has been provided to the receiver outlet tube 18b be omitted, and a third expansion mechanism for depressurizing the refrigerant to a low pressure in the refrigeration cycle during the air-warming operation be provided instead of the outlet non-return valve 17d of the bridge circuit 17. 25 In this type of configuration as well, the intermediate pressure injection by the economizer heat exchanger 20 is advantageous similar to Modification 3 described above, under the condition that the pressure difference between the high pressure in the refrigeration cycle and the nearly intermediate pressure of the refrigeration cycle can be used without performing a severe depressurizing operation everywhere but the first expansion mechanism 30 5a as a heat source-side expansion mechanism after cooling takes place in the heat source side heat exchanger 4 as a radiator, as is the case in the air-cooling operation in which the switching mechanism 3 is set to the cooling operation state. However, in conditions such as those of the air-warming operation for setting the switching mechanism 3 to the heating operation state, the usage-side expansion mechanisms 39 5c control the flow rate of refrigerant flowing through the usage-side heat exchangers 6 as radiators so as to obtain the refrigeration loads required by the usage-side heat exchangers 6 as radiators, and the flow rate of refrigerant passing through the usage-side heat exchangers 6 as radiators is established for the most part by the operation of depressurizing the refrigerant 5 by controlling the opening degrees of the usage-side expansion mechanisms 5c provided downstream of the usage-side heat exchangers 6 as radiators and upstream of the economizer heat exchanger 20. Under such conditions, the extent of refrigerant depressurization by controlling the opening degrees of the usage-side expansion mechanisms 5c fluctuates not only due to the flow rate of refrigerant flowing through the usage-side heat exchangers 6 as 10 radiators, but also due to the state of flow rate distribution among the plurality of usage-side heat exchangers 6 as radiators, and there are cases in which the extent of depressurization differs greatly among the plurality of usage-side expansion mechanisms 5c, or the extent of depressurization in the usage-side expansion mechanisms 5c is comparatively large. Therefore, there is a risk of a decrease in the refrigerant pressure in the inlet of the 15 economizer heat exchanger 20, and in such cases there is a risk that the quantity of heat exchanged in the economizer heat exchanger 20 (i.e., the flow rate of refrigerant flowing through the first second-stage injection tube 19) will decrease and usage will be difficult. Particularly in cases in which this type of air-conditioning apparatus I is configured as a separate-type air-conditioning apparatus in which a heat source unit including primarily a 20 compression mechanism 2, a heat source-side heat exchanger 4, and a receiver 18 is connected by communication pipe with a usage unit including primarily a usage-side heat exchanger 6, the communication pipe might be extremely long depending on the arrangement of the usage unit and the heat source unit; therefore, in addition to the effects of pressure drop, the pressure of the refrigerant in the inlet of the economizer heat exchanger 20 further 25 decreases. In cases in which there is a risk of a decrease in the pressure of the refrigerant in the inlet of the economizer heat exchanger 20, if the gas-liquid separator pressure is lower than the critical pressure, intermediate pressure injection by a useable gas-liquid separator is still advantageous even under conditions in which there is a small difference in pressure between the gas-liquid separator pressure and the intermediate pressure in the refrigeration 30 cycle (here, the pressure of the refrigerant flowing through the intermediate refrigerant tube 8). In view of this, in the present modification as shown in FIG. 19, to enable the receiver 18 to function as a gas-liquid separator and to enable intermediate pressure injection to be performed, a refrigerant circuit 310 is used in which a second second-stage injection 40 tube 18c is connected to the receiver 18, intermediate pressure injection can be performed by the economizer heat exchanger 20 during the air-cooling operation, and intermediate pressure injection can be performed by the receiver 18 as a gas-liquid separator during the air warming operation. 5 The second second-stage injection tube 18c is a refrigerant tube capable of performing intermediate pressure injection for removing refrigerant from the receiver 18 and returning the refrigerant to the second-stage compression element 2d of the compression mechanism 2, and in the present modification, the second second-stage injection tube 18c is provided so as to connect the top part of the receiver 18 and the intermediate refrigerant tube 10 8 (i.e., the intake side of the second-stage compression element 2d of the compression mechanism 2). The second second-stage injection tube 18c is provided with a second second-stage injection on/off valve 18d and a second second-stage injection non-return mechanism 18e. The second second-stage injection on/off valve 18d is a valve capable of opening and closing, and is an electromagnetic valve in the present modification. The 15 second second-stage injection non-return mechanism 18e is a mechanism for allowing refrigerant to flow from the receiver 18 to the second-stage compression element 2d and blocking refrigerant from flowing from the second-stage compression element 2d to the receiver 18, and a non-return valve is used in the present modification. The second second stage injection tube 18c and the first intake return tube 18f are integrated in a portion near the 20 receiver 18. The second second-stage injection tube 18c and the first second-stage injection tube 19 are integrated in a portion near the intermediate refrigerant tube 8. In the present modification, the usage-side expansion mechanisms 5c are electrically driven expansion valves. In the present modification, as described above, the first second-stage injection tube 19 and the economizer heat exchanger 20 are used during the air-cooling operation, and the 25 second second-stage injection tube 18c is used during the air-warming operation; therefore, since there is no need for the direction of refrigerant flow to the economizer heat exchanger 20 to be constant for both the air-cooling operation and the air-warming operation, the bridge circuit 17 is omitted and the configuration of the refrigerant circuit 310 is simplified. Next, the action of the air-conditioning apparatus I of the present modification will 30 be described using FIGS. 19, 15, 16, 20, and 21. FIG. 20 is a pressure-enthalpy graph representing the refrigeration cycle during the air-warming operation, and FIG. 21 is a temperature-entropy graph representing the refrigeration cycle during the air-warming operation. This air-cooling start control is the same as that of the embodiment described above and is therefore not described herein. The refrigeration cycle during the air-cooling 41 operation in the present modification is described using FIGS. 15 and 16. Operation control (including air-cooling start control not described herein) in the following air-cooling operation and air-warming operation is performed by the controller (not shown) in the embodiment described above. In the following description, the term "high pressure" means 5 a high pressure in the refrigeration cycle (specifically, the pressure at points D, D', E, and H in FIGS. 15 and 16, and the pressure at points D, D', and F in FIGS. 20 and 21), the term "low pressure" means a low pressure in the refrigeration cycle (specifically, the pressure at points A and F in FIGS. 15 and 16, and the pressure at points A, E, and V in FIGS. 20 and 21), and the term "intermediate pressure" means an intermediate pressure in the refrigeration 10 cycle (specifically, the pressure at points BI, C1, G, J, and K in FIGS. 15 and 16, and the pressure at points BI, C1, G, I, L, and M in FIGS. 20 and 21). <Air-cooling operation> During the air-cooling operation, the switching mechanism 3 is brought to the cooling operation state shown by the solid lines in FIG 19. The opening degrees of the first 15 expansion mechanism 5a and the usage-side expansion mechanisms 5c as heat source-side expansion mechanisms are adjusted. Since the switching mechanism 3 is in the cooling operation state, opening the intermediate heat exchanger on/off valve 12 of the intermediate refrigerant tube 8 and closing the intermediate heat exchanger bypass on/off valve 11 of the intermediate heat exchanger bypass tube 9 creates a state in which the intermediate heat 20 exchanger 7 functions as a cooler, closing the second intake return on/off valve 92a of the second intake return tube 92 creates a state in which the intermediate heat exchanger 7 and the intake side of the compression mechanism 2 are not connected (except during air-cooling start control), and closing the intermediate heat exchanger return on/off valve 94a of the intermediate heat exchanger return tube 94 creates a state in which the portion between the 25 usage-side heat exchangers 6 and the heat source-side heat exchanger 4 is not connected with the intermediate heat exchanger 7. When the switching mechanism 3 has been set to the cooling operation state, intermediate pressure injection is not performed by the receiver 18 as a gas-liquid separator, but instead intermediate pressure injection is performed by the economizer heat exchanger 20 for returning the refrigerant heated in the economizer heat 30 exchanger 20 to the second-stage compression element 2d through the first second-stage injection tube 19. More specifically, the second second-stage injection on/off valve 18d is closed, and the opening degree of the first second-stage injection valve 19a is adjusted in the same manner as Modification 3 described above. When the refrigerant circuit 310 is in this state, low-pressure refrigerant (refer to 42 point A in FIGS. 19, 15, and 16) is drawn into the compression mechanism 2 through the intake tube 2a, and after the refrigerant is first compressed to an intermediate pressure by the compression element 2c, the refrigerant is discharged to the intermediate refrigerant tube 8 (refer to point B1 in FIGS. 19, 15, 16). The intermediate-pressure refrigerant discharged 5 from the first-stage compression element 2c is cooled by heat exchange with water or air as a cooling source in the intermediate heat exchanger 7 (refer to point C1 in FIGS. 19, 15, and 16). The refrigerant cooled in the intermediate heat exchanger 7 is further cooled (refer to point G in FIGS. 19, 15, and 16) by being mixed with refrigerant being returned from the first second-stage injection tube 19 to the second-stage compression element 2d (refer to point K 10 in FIGS. 19, 15, and16). Next, having been mixed with the refrigerant returning from the first second-stage injection tube 19 (i.e., intermediate pressure injection is carried out by the economizer heat exchanger 20), the intermediate-pressure refrigerant is drawn into and further compressed in the compression element 2d connected to the second-stage side of the compression element 2c, and the refrigerant is discharged from the compression mechanism 2 15 to the discharge tube 2b (refer to point D in FIGS. 19, 15, and 16). The high-pressure refrigerant discharged from the compression mechanism 2 is compressed by the two-stage compression action of the compression elements 2c, 2d to a pressure exceeding a critical pressure (i.e., the critical pressure Pep at the critical point CP shown in FIG. 15). The high pressure refrigerant discharged from the compression mechanism 2 is fed via the switching 20 mechanism 3 to the heat source-side heat exchanger 4 functioning as a refrigerant radiator, and the refrigerant is cooled by heat exchange with water or air as a cooling source (refer to point E in FIGS. 19, 15, and 16). Some of the high-pressure refrigerant cooled in the heat source-side heat exchanger 4 as a radiator is then branched off to the first second-stage injection tube 19. The refrigerant flowing through the first second-stage injection tube 19 is 25 depressurized to a nearly intermediate pressure in the first second-stage injection valve 19a and is then fed to the economizer heat exchanger 20 (refer to point J in FIGS. 19, 15, and 16). The refrigerant branched off to the first second-stage injection tube 19 then flows into the economizer heat exchanger 20, where it is cooled by heat exchange with the refrigerant flowing through the first second-stage injection tube 19 (refer to point H in FIGS. 19, 15, and 30 16). The refrigerant flowing through the first second-stage injection tube 19 is heated by heat exchange with the high-pressure refrigerant cooled in the heat source-side heat exchanger 4 as a radiator (refer to point K in FIGS. 19, 15, and 16), and is mixed with the intermediate-pressure refrigerant discharged from the first-stage compression element 2c as described above. The high-pressure refrigerant cooled in the economizer heat exchanger 20 43 is depressurized to a nearly saturated pressure by the first expansion mechanism 5a and is temporarily retained in the receiver 18 (refer to point I in FIGS. 19, 15, and 16). The refrigerant retained in the receiver 18 is then fed to the usage-side expansion mechanisms 5c and depressurized by the usage-side expansion mechanisms 5c to become a low-pressure gas 5 liquid two-phase refrigerant, which is fed to the usage-side heat exchangers 6 functioning as evaporators of refrigerant (refer to point F in FIGS. 19, 15, and 16). The low-pressure gas liquid two-phase refrigerant fed to the usage-side heat exchangers 6 that function as evaporators is heated by heat exchange with water or air as a heating source, and the refrigerant is evaporated as a result (refer to point A in FIGS. 19, 15, and 16). The low 10 pressure refrigerant heated and evaporated in the usage-side heat exchangers 6 that function evaporators is then drawn back into the compression mechanism 2 via the switching mechanism 3. In this manner the air-cooling operation is performed. <Air-warming operation> During the air-warming operation, the switching mechanism 3 is brought to the 15 heating operation state shown by the dashed lines in FIG. 19. The opening degrees of the first expansion mechanism 5a and the usage-side expansion mechanisms 5c as heat source side expansion mechanisms are adjusted. Since the switching mechanism 3 is set to the heating operation state, the intermediate heat exchanger on/off valve 12 of the intermediate refrigerant tube 8 is closed and the intermediate heat exchanger bypass on/off valve 11 of the 20 intermediate heat exchanger bypass tube 9 is opened, thereby putting the intermediate heat exchanger 7 into a state of not functioning as a cooler. Furthermore, since the switching mechanism 3 is in the heating operation state, opening the second intake return on/off valve 92a of the second intake return tube 92 creates a state in which the intermediate heat exchanger 7 and the intake side of the compression mechanism 2 are connected, and opening 25 the intermediate heat exchanger return on/off valve 94a of the intermediate heat exchanger return tube 94 creates a state in which the portion between the usage-side heat exchangers 6 and the heat source-side heat exchanger 4 is connected with the intermediate heat exchanger 7. When the switching mechanism 3 has been set to the heating operation state, intermediate pressure injection is not performed by the economizer heat exchanger 20, but 30 instead intermediate pressure injection is performed by the receiver 18 for returning the refrigerant from the receiver 18 as a gas-liquid separator to the second-stage compression element 2d through the second second-stage injection tube 18c. More specifically, the second second-stage injection on/off valve 18d is opened, and the first second-stage injection valve 19a is fully closed. 44 When the refrigerant circuit 310 is in this state, low-pressure refrigerant (refer to point A in FIGS. 19 through 21) is drawn into the compression mechanism 2 through the intake tube 2a, and after the refrigerant is first compressed to an intermediate pressure by the compression element 2c, the refrigerant is discharged to the intermediate refrigerant tube 8 5 (refer to point BI in FIGS. 19 through 21). Unlike during the air-cooling operation, the intermediate-pressure refrigerant discharged from the first-stage compression element 2c passes through the intermediate heat exchanger bypass tube 9 (refer to point Cl in FIGS. 19 to 21) without passing through the intermediate heat exchanger 7 (i.e., without being cooled), and is mixed with the refrigerant returning from the receiver 18 to the second-stage 10 compression element 2d through the second second-stage injection tube 18c (refer to point M in FIGS. 19 to 21), thereby being cooled (refer to point G in FIGS. 19 to 21). Next, having been mixed with the refrigerant returning from the second second-stage injection tube 18c (i.e., intermediate pressure injection is carried out by the receiver 18 which acts as a gas liquid separator), the intermediate-pressure refrigerant is drawn into and further compressed 15 in the compression element 2d connected to the second-stage side of the compression element 2c, and the refrigerant is discharged from the compression mechanism 2 to the discharge tube 2b (refer to point D in FIGS. 19 through 21). The high-pressure refrigerant discharged from the compression mechanism 2 is compressed by the two-stage compression action of the compression elements 2c, 2d to a pressure exceeding a critical pressure (i.e., the critical 20 pressure Pcp at the critical point CP shown in FIG. 20), similar to the air-cooling operation. The high-pressure refrigerant discharged from the compression mechanism 2 is fed via the switching mechanism 3 to the usage-side heat exchangers 6 functioning as refrigerant radiators, and the refrigerant is cooled by heat exchange with water or air as a cooling source (refer to point F in FIGS. 19 to 21). The high-pressure refrigerant cooled in the usage-side 25 heat exchangers 6 as radiators is then depressurized to a nearly intermediate pressure by the usage-side expansion mechanisms 5c, and is then temporarily retained in the receiver 18 and subjected to gas-liquid separation (refer to points I, L, and M in FIGS. 19 to 21). The gas refrigerant after having undergone gas-liquid separation in the receiver 18 is then removed from the top part of the receiver 18 by the second second-stage injection tube 18c, and is 30 mixed with the intermediate-pressure refrigerant discharged from the first-stage compression element 2c as described above. The liquid refrigerant retained in the receiver 18 is depressurized by the first expansion mechanism 5a to become a low-pressure gas-liquid two phase refrigerant, which is fed to the heat source-side heat exchanger 4 functioning as an evaporator of refrigerant, and is also fed through the intermediate heat exchanger return tube 45 94 to the intermediate heat exchanger 7 functioning as an evaporator of refrigerant (refer to point E in FIGS. 19 to 21). The low-pressure gas-liquid two-phase refrigerant fed to the heat source-side heat exchanger 4 is heated by heat exchange with water or air as a heating source, and the refrigerant is evaporated as a result (refer to point A in FIGS. 19 to 21). The 5 low-pressure gas-liquid two-phase refrigerant fed to the intermediate heat exchanger 7 is also heated by heat exchange with water or air as a heating source, and the refrigerant evaporates as a result (refer to point V in FIGS. 19 through 21). The low-pressure refrigerant heated and evaporated in the heat source-side heat exchanger 4 is then drawn back into the compression mechanism 2 via the switching mechanism 3. The low-pressure refrigerant 10 heated and evaporated in the intermediate heat exchanger 7 is then drawn back into the compression mechanism 2 via the second intake return tube 92. In this manner the air warming operation is performed. The configuration of the present modification is different from that of Modification 3 in that intermediate pressure injection is performed by the receiver 18 as a gas-liquid 15 separator instead of intermediate pressure injection by the economizer heat exchanger 20 during the air-warming operation, but otherwise the same operational effects as those of Modification 3 can be obtained. In the present modification, the switching between the air-cooling operation and air cooling start control, i.e., the switching between the refrigerant non-return state and the 20 refrigerant return state is performed by the on/off states of the on/off valves 11, 12, 92a, but an intermediate heat exchanger switching valve 93 capable of switching between the refrigerant non-return state and the refrigerant return state may be provided instead of the on/off valves 11, 12, 92a, as in Modification 1 described above. Furthermore, particularly advantageous effects can be obtained when using the 25 configuration of the heat source unit la such as the one in Modification 2. (7) Modification 5 The refrigerant circuit 310 (see FIG. 19) in Modification 4 described above comprises a configuration having a plurality of usage-side heat exchangers 6 connected to each other in parallel with the objective of performing air cooling and/or air warming 30 according to the air-conditioning load of a plurality of air-conditioned spaces, for example, and also uses a configuration in which usage-side expansion mechanisms 5c are provided between the receiver 18 and the usage-side heat exchangers 6 so as to correspond to the usage-side heat exchangers 6, in order to make it possible to control the flow rate of refrigerant flowing through the usage-side heat exchangers 6 and obtain the refrigeration load 46 required by the usage-side heat exchangers 6. With this type of configuration, during the air-cooling operation, the refrigerant depressurized to a nearly saturated pressure by the first expansion mechanism Sa and temporarily retained in the receiver 18 (refer to point I in FIG 19) is distributed among the usage-side expansion mechanisms 5c, but when the refrigerant 5 fed from the receiver 18 to the usage-side expansion mechanisms 5c is in a gas-liquid two phase state, there is a risk of the flow being imbalanced when the refrigerant is distributed among the usage-side expansion mechanisms 5c, and it is therefore preferable that the refrigerant fed from the receiver 18 to the usage-side expansion mechanisms 5c be brought as much as possible to a subcooled state. 10 In view of this, in the present modification, the refrigerant circuit 310 in Modification 4 described above is replaced by a refrigerant circuit 410 provided with a subcooling heat exchanger 96 and a third intake return tube 95 between the receiver 18 and the usage-side expansion mechanisms 5c, as shown in FIG 22. The subcooling heat exchanger 96 is a heat exchanger for cooling the refrigerant fed 15 from the receiver 18 to the usage-side expansion mechanisms 5c. More specifically, during the air-cooling operation, the subcooling heat exchanger 96 is a heat exchanger for performing heat exchange with the refrigerant flowing through the third intake return tube 95, which branches off some of the refrigerant fed from the receiver 18 to the usage-side expansion mechanisms 5c and returns the refrigerant to the intake side of the compression 20 mechanism 2 (i.e., to the intake tube 2a between the compression mechanism 2 and the usage-side heat exchangers 6 as evaporators), and the subcooling heat exchanger 96 has a flow passage through which both refrigerants flow against each other. The third intake return tube 95 herein is a refrigerant tube for branching off the refrigerant fed from the heat source-side heat exchanger 4 as a radiator to the usage-side expansion mechanisms 5c and 25 returning the refrigerant to the intake side of the compression mechanism 2 (i.e., the intake tube 2a). The third intake return tube 95 is provided with a third intake return valve 95a whose opening degree can be controlled, and in the subcooling heat exchanger 96, heat exchange is performed between the refrigerant fed from the receiver 18 to the usage-side expansion mechanisms 5c and the refrigerant flowing through the third intake return tube 95 30 after having been depressurized to a nearly low pressure in the third intake return valve 95a. The intake return valve 95a is an electrically driven expansion valve in the present modification. An intake pressure sensor 60 for detecting the pressure of the refrigerant flowing through the intake side of the compression mechanism 2 is provided to either the intake tube 2a or the compression mechanism 2. The outlet of the subcooling heat 47 exchanger 96 on the side near the third intake return tube 95 is provided with a subcooling heat exchange outlet temperature sensor 59 for detecting the temperature of the refrigerant in the outlet of the subcooling heat exchanger 96 on the side near the third intake return tube 95. Next, the action of the air-conditioning apparatus I in the present modification will 5 be described using FIGS. 22 to 24, 20, and 21. FIG. 23 is a pressure-enthalpy graph representing the refrigeration cycle during the air-cooling operation, and FIG. 24 is a temperature-entropy graph representing the refrigeration cycle during the air-cooling operation. This air-cooling start control is the same as that of the embodiment described above and is therefore not described herein. The refrigeration cycle during the air-warming 10 operation in the present modification is described using FIGS. 20 and 21. Operation control during the following air-cooling operation and air-warming operation (including the air cooling start control not described herein) is performed by the controller (not shown) in the embodiment described above. In the following description, the term "high pressure" means a high pressure in the refrigeration cycle (specifically, the pressure at points D, E, I, and R in 15 FIGS. 23 and 24, and the pressure at points D, D', and F in FIGS. 20 and 21), the term "low pressure" means a low pressure in the refrigeration cycle (specifically, the pressure at points A, F, F, S', and U in FIGS. 23 and 24, and the pressure at points A, E, and V in FIGS. 20 and 21), and the term "intermediate pressure" means an intermediate pressure in the refrigeration cycle (specifically, the pressure at points B1, Cl, G, J, and K in FIGS. 23 and 24, and the 20 pressure at points BI, C1, G, I, L, and M in FIGS. 20 and 21). <Air-cooling operation> During the air-cooling operation, the switching mechanism 3 is brought to the cooling operation state shown by the solid lines in FIG. 22. The opening degrees of the first expansion mechanism 5a and the usage-side expansion mechanisms 5c as heat source-side 25 expansion mechanisms are adjusted. Since the switching mechanism 3 is in the cooling operation state, opening the intermediate heat exchanger on/off valve 12 of the intermediate refrigerant tube 8 and closing the intermediate heat exchanger bypass on/off valve 11 of the intermediate heat exchanger bypass tube 9 creates a state in which the intermediate heat exchanger 7 functions as a cooler, closing the second intake return on/off valve 92a of the 30 second intake return tube 92 creates a state in which the intermediate heat exchanger 7 and the intake side of the compression mechanism 2 are not connected (except during air-cooling start control), and closing the intermediate heat exchanger return on/off valve 94a of the intermediate heat exchanger return tube 94 creates a state in which the portion between the usage-side heat exchangers 6 and the heat source-side heat exchanger 4 is not connected with 48 the intermediate heat exchanger 7. When the switching mechanism 3 has been set to the cooling operation state, intermediate pressure injection is not performed by the receiver 18 as a gas-liquid separator, but instead intermediate pressure injection is performed by the economizer heat exchanger 20 for returning the refrigerant heated in the economizer heat 5 exchanger 20 to the second-stage compression element 2d through the first second-stage injection tube 19. More specifically, the second second-stage injection on/off valve 18d is closed, and the opening degree of the first second-stage injection valve 19a is adjusted in the same manner as Modification 3 described above. When the switching mechanism 3 is in the cooling operation state, the opening degree of the third intake return valve 95a is also 10 adjusted because the subcooling heat exchanger 96 is used. More specifically, in the present modification, what is known as superheat degree control is performed, wherein the opening degree of the third intake return valve 95a is adjusted so that a target value is achieved in the degree of superheat of the refrigerant at the outlet in the third intake return tube 95 side of the subcooling heat exchanger 96. In the present modification, the degree of superheat of the 15 refrigerant at the outlet in the third intake return tube 95 side of the subcooling heat exchanger 96 is obtained by converting the low pressure detected by the intake pressure sensor 60 to a saturation temperature and subtracting this refrigerant saturation temperature value from the refrigerant temperature detected by the subcooling heat exchanger outlet temperature sensor 59. Though not used in the present modification, another possible 20 option is to provide a temperature sensor to the inlet in the third intake return tube 95 side of the subcooling heat exchanger 96, and to obtain the degree of superheat of the refrigerant at the outlet in the third intake return tube 95 side of the subcooling heat exchanger 96 by subtracting the refrigerant temperature detected by this temperature sensor from the refrigerant temperature detected by the subcooling heat exchanger outlet temperature sensor 25 59. Adjusting the opening degree of the third intake return valve 95a is not limited to the superheat degree control, and the third intake return valve 95a may be opened to a predetermined opening degree in accordance with the flow rate of refrigerant circulating within the refrigerant circuit 410, for example. When the refrigerant circuit 410 is in this state, low-pressure refrigerant (refer to 30 point A in FIGS. 22 through 24) is drawn into the compression mechanism 2 through the intake tube 2a, and after the refrigerant is first compressed to an intermediate pressure by the compression element 2c, the refrigerant is discharged to the intermediate refrigerant tube 8 (refer to point BI in FIGS. 22 through 24). The intermediate-pressure refrigerant discharged from the first-stage compression element 2c is cooled by heat exchange with 49 water or air as a cooling source in the intermediate heat exchanger 7 (refer to point C1 in FIGS. 22 to 24). The refrigerant cooled in the intermediate heat exchanger 7 is further cooled (refer to point G in FIGS. 22 to 24) by being mixed with refrigerant being returned from the first second-stage injection tube 19 to the second-stage compression element 2d 5 (refer to point K in FIGS. 22 to 24). Next, having been mixed with the refrigerant returning from the first second-stage injection tube 19 (i.e., intermediate pressure injection is carried out by the economizer heat exchanger 20), the intermediate-pressure refrigerant is drawn into and further compressed in the compression element 2d connected to the second-stage side of the compression element 2c, and the refrigerant is discharged from the compression 10 mechanism 2 to the discharge tube 2b (refer to point D in FIGS. 22 through 24). The high pressure refrigerant discharged from the compression mechanism 2 is compressed by the two stage compression action of the compression elements 2c, 2d to a pressure exceeding a critical pressure (i.e., the critical pressure Pcp at the critical point CP shown in FIG. 23). The high-pressure refrigerant discharged from the compression mechanism 2 is fed via the 15 switching mechanism 3 to the heat source-side heat exchanger 4 functioning as a refrigerant radiator, and the refrigerant is cooled by heat exchange with water or air as a cooling source (refer to point E in FIGS. 22 to 24). Some of the high-pressure refrigerant cooled in the heat source-side heat exchanger 4 as a radiator is then branched off to the first second-stage injection tube 19. The refrigerant flowing through the first second-stage injection tube 19 is 20 depressurized to a nearly intermediate pressure in the first second-stage injection valve 19a and is then fed to the economizer heat exchanger 20 (refer to point J in FIGS. 22 to 24). The refrigerant branched off to the first second-stage injection tube 19 then flows into the economizer heat exchanger 20, where it is cooled by heat exchange with the refrigerant flowing through the first second-stage injection tube 19 (refer to point H in FIGS. 20 to 22). 25 The refrigerant flowing through the first second-stage injection tube 19 is heated by heat exchange with the high-pressure refrigerant cooled in the heat source-side heat exchanger 4 as a radiator (refer to point K in FIGS. 22 to 24), and is mixed with the intermediate-pressure refrigerant discharged from the first-stage compression element 2c as described above. The high-pressure refrigerant cooled in the economizer heat exchanger 20 is depressurized to a 30 nearly saturated pressure by the first expansion mechanism 5a and is temporarily retained in the receiver 18 (refer to point I in FIGS. 22 to 24). Some of the refrigerant retained in the receiver 18 is then branched off to the third intake return tube 95. The refrigerant flowing through the third intake return tube 95 is depressurized to a nearly low pressure in the third intake return valve 95a and is then fed to the subcooling heat exchanger 96 (refer to point S in 50 FIGS. 20 to 22). The refrigerant branched off to the third intake return tube 95 then flows into the subcooling heat exchanger 96, where it is further cooled by heat exchange with the refrigerant flowing through the third intake return tube 95 (refer to point R in FIGS. 22 to 24). The refrigerant flowing through the third intake return tube 95 is heated by heat 5 exchange with the high-pressure refrigerant cooled in the economizer heat exchanger 20 (refer to point U in FIGS. 22 to 24), and is mixed with the refrigerant flowing through the intake side of the compression mechanism 2 (here, the intake tube 2a). The refrigerant cooled in the subcooling heat exchanger 96 is then fed to the usage-side expansion mechanisms 5c and depressurized by the usage-side expansion mechanisms 5c to become a 10 low-pressure gas-liquid two-phase refrigerant, and is then fed to the usage-side heat exchangers 6 functioning as evaporators of refrigerant (refer to point F in FIGS. 22 to 24). The low-pressure gas-liquid two-phase refrigerant fed to the usage-side heat exchangers 6 that function as evaporators is heated by heat exchange with water or air as a heating source, and the refrigerant is evaporated as a result (refer to point A in FIGS. 22 to 24). The low 15 pressure refrigerant heated and evaporated in the usage-side heat exchangers 6 that function as evaporators is then drawn back into the compression mechanism 2 via the switching mechanism 3. In this manner the air-cooling operation is performed. <Air-warming operation> During the air-warming operation, the switching mechanism 3 is brought to the 20 heating operation state shown by the dashed lines in FIG. 22. The opening degrees of the first expansion mechanism 5a and the usage-side expansion mechanisms 5c as heat source side expansion mechanisms are adjusted. Since the switching mechanism 3 is set to the heating operation state, the intermediate heat exchanger on/off valve 12 of the intermediate refrigerant tube 8 is closed and the intermediate heat exchanger bypass on/off valve 11 of the 25 intermediate heat exchanger bypass tube 9 is opened, thereby putting the intermediate heat exchanger 7 into a state of not functioning as a cooler. Furthermore, since the switching mechanism 3 is in the heating operation state, opening the second intake return on/off valve 92a of the second intake return tube 92 creates a state in which the intermediate heat exchanger 7 and the intake side of the compression mechanism 2 are connected, and opening 30 the intermediate heat exchanger return on/off valve 94a of the intermediate heat exchanger return tube 94 creates a state in which the portion between the usage-side heat exchangers 6 and the heat source-side heat exchanger 4 is connected with the intermediate heat exchanger 7. When the switching mechanism 3 has been set to the heating operation state, intermediate pressure injection is not performed by the economizer heat exchanger 20, but 51 instead intermediate pressure injection is performed by the receiver 18 for returning the refrigerant from the receiver 18 as a gas-liquid separator to the second-stage compression element 2d through the second second-stage injection tube 18c. More specifically, the second second-stage injection on/off valve 18d is opened, and the first second-stage injection 5 valve 19a is fully closed. When the switching mechanism 3 has been set to the heating operation state, the third intake return valve 95a is also fully closed because the subcooling heat exchanger 96 is not used. When the refrigerant circuit 410 is in this state, low-pressure refrigerant (refer to point A in FIGS. 22, 20, and 21) is drawn into the compression mechanism 2 through the 10 intake tube 2a, and after the refrigerant is first compressed to an intermediate pressure by the compression element 2c, the refrigerant is discharged to the intermediate refrigerant tube 8 (refer to point BI in FIGS. 22, 20, 21). Unlike the air-cooling operation, the intermediate pressure refrigerant discharged from the first-stage compression element 2c passes through the intermediate heat exchanger bypass tube 9 (refer to point C1 in FIGS. 22, 20, and 21) 15 without passing through the intermediate heat exchanger 7 (i.e., without being cooled), and the refrigerant is cooled (refer to point G in FIGS. 22, 20, and 21) by being mixed with refrigerant being returned from the receiver 18 via the second second-stage injection tube 18c to the second-stage compression element 2d (refer to point M in FIGS. 22, 20, and 21). Next, having been mixed with the refrigerant returning from the second second-stage 20 injection tube 18c (i.e., intermediate pressure injection is carried out by the receiver 18 which acts as a gas-liquid separator), the intermediate-pressure refrigerant is drawn into and further compressed in the compression element 2d connected to the second-stage side of the compression element 2c, and the refrigerant is discharged from the compression mechanism 2 to the discharge tube 2b (refer to point D in FIGS. 22, 20, and 21). The high-pressure 25 refrigerant discharged from the compression mechanism 2 is compressed by the two-stage compression action of the compression elements 2c, 2d to a pressure exceeding a critical pressure (i.e., the critical pressure Pcp at the critical point CP shown in FIG. 20), similar to the air-cooling operation. The high-pressure refrigerant discharged from the compression mechanism 2 is fed via the switching mechanism 3 to the usage-side heat exchangers 6 30 functioning as refrigerant radiators, and the refrigerant is cooled by heat exchange with water or air as a cooling source (refer to point F in FIGS. 22, 20, and 21). The high-pressure refrigerant cooled in the usage-side heat exchangers 6 as radiators is then depressurized to a nearly intermediate pressure by the usage-side expansion mechanisms 5c, and is then temporarily retained in the receiver 18 and subjected to gas-liquid separation (refer to points 52 I, L, and M in FIGS. 22, 20, and 21). The gas refrigerant after having undergone gas-liquid separation in the receiver 18 is then removed out of the top part of the receiver 18 by the second second-stage injection tube 18c, and is mixed with the intermediate-pressure refrigerant discharged from the first-stage compression element 2c as described above. The 5 liquid refrigerant retained in the receiver 18 is depressurized by the first expansion mechanism 5a to become a low-pressure gas-liquid two-phase refrigerant, which is fed to the heat source-side heat exchanger 4 functioning as an evaporator of refrigerant, and is also fed through the intermediate heat exchanger return tube 94 to the intermediate heat exchanger 7 functioning as an evaporator of refrigerant (refer to point E in FIGS. 22, 20, and 21). The 10 low-pressure gas-liquid two-phase refrigerant fed to the heat source-side heat exchanger 4 is heated by heat exchange with water or air as a heating source, and the refrigerant evaporates as a result (refer to point A in FIGS. 22, 20, and 21). The low-pressure gas-liquid two-phase refrigerant fed to the intermediate heat exchanger 7 is also heated by heat exchange with water or air as a heating source, and the refrigerant evaporates as a result (refer to point V in 15 FIGS. 22, 20, and 21). The low-pressure refrigerant heated and evaporated in the heat source-side heat exchanger 4 is then drawn back into the compression mechanism 2 via the switching mechanism 3. The low-pressure refrigerant heated and evaporated in the intermediate heat exchanger 7 is then drawn back into the compression mechanism 2 via the second intake return tube 92. In this manner the air-warming operation is performed. 20 In the configuration of the present modification, the same operational effects as those of Modification 5 described above are achieved, and the risk of an imbalanced flow of refrigerant during distribution to the usage-side expansion mechanisms 5c can be reduced because the refrigerant fed from the receiver 18 to the usage-side expansion mechanisms 5c during the air-cooling operation (refer to point I in FIGS. 22 to 24) can be cooled to a 25 subcooled state by the subcooling heat exchanger 96 (refer to points I and R in FIGS. 23 and 24). In the present modification, the switching between the air-cooling operation and air cooling start control, i.e., the switching between the refrigerant non-return state and the refrigerant return state is performed by the on/off states of the on/off valves 11, 12, 92a, but 30 an intermediate heat exchanger switching valve 93 capable of switching between the refrigerant non-return state and the refrigerant return state may be provided instead of the on/off valves 11, 12, 92a, as in Modification 1 described above. Furthermore, particularly advantageous effects can be obtained when using the configuration of the heat source unit la such as the one in Modification 2. 53 (8) Modification 6 In the above-described embodiment and the modifications thereof, a two-stage compression-type compression mechanism 2 is configured such that the refrigerant discharged from the first-stage compression element of two compression elements 2c, 2d is 5 sequentially compressed in the second-stage compression element by one compressor 21 having a single-axis two-stage compression structure, but other options include using a compression mechanism having more stages than a two-stage compression system, such as a three-stage compression system or the like; or configuring a multistage compression mechanism connecting in series a plurality of compressors incorporated with a single 10 compression element and/or compressors incorporated with a plurality of compression elements. In cases in which the capacity of the compression mechanism must be increased, such as cases in which numerous usage-side heat exchangers 6 are connected, for example, a parallel multistage compression-type compression mechanism may be used in which two or more multistage compression-type compression mechanisms are connected in parallel. 15 For example, the refrigerant circuit 410 in Modification 5 described above (see FIG 22) may be replaced by a refrigerant circuit 510 that uses a compression mechanism 102 in which two-stage compression-type compression mechanisms 103, 104 are connected in parallel instead of the two-stage compression-type compression mechanism 2, as shown in FIG 25. 20 In the present modification, the first compression mechanism 103 is configured using a compressor 29 for subjecting the refrigerant to two-stage compression through two compression elements 103c, 103d, and is connected to a first intake branch tube 103a which branches off from an intake header tube 102a of the compression mechanism 102, and also to a first discharge branch tube 103b whose flow merges with a discharge header tube 102b of 25 the compression mechanism 102. In the present modification, the second compression mechanism 104 is configured using a compressor 30 for subjecting the refrigerant to two stage compression through two compression elements 104c, 104d, and is connected to a second intake branch tube 104a which branches off from the intake header tube 102a of the compression mechanism 102, and also to a second discharge branch tube 104b whose flow 30 merges with the discharge header tube 102b of the compression mechanism 102. Since the compressors 29, 30 have the same configuration as the compressor 21 in the embodiment and modifications thereof described above, symbols indicating components other than the compression elements 103c, 103d, 104c, 104d are replaced with symbols beginning with 29 or 30, and these components are not described here. The compressor 29 is configured so 54 that refrigerant is drawn from the first intake branch tube 103a, the refrigerant thus drawn in is compressed by the compression element 103c and then discharged to a first inlet-side intermediate branch tube 81 that constitutes the intermediate refrigerant tube 8, the refrigerant discharged to the first inlet-side intermediate branch tube 81 is caused to be drawn into the 5 compression element 103d by way of an intermediate header tube 82 and a first outlet-side intermediate branch tube 83 constituting the intermediate refrigerant tube 8, and the refrigerant is further compressed and then discharged to the first discharge branch tube 103b. The compressor 30 is configured so that refrigerant is drawn in through the second intake branch tube 104a, the drawn-in refrigerant is compressed by the compression element 104c 10 and then discharged to a second inlet-side intermediate branch tube 84 constituting the intermediate refrigerant tube 8, the refrigerant discharged to the second inlet-side intermediate branch tube 84 is drawn into the compression element 104d via the intermediate header tube 82 and a second outlet-side intermediate branch tube 85 constituting the intermediate refrigerant tube 8, and the refrigerant is further compressed and then discharged 15 to the second discharge branch tube 104b. In the present modification, the intermediate refrigerant tube 8 is a refrigerant tube for drawing in refrigerant discharged from the compression elements 103c, 104c connected to the first-stage sides of the compression elements 103d, 104d into the compression elements 103d, 104d connected to the second stage sides of the compression elements 103c, 104c, and the intermediate refrigerant tube 8 20 primarily comprises the first inlet-side intermediate branch tube 81 connected to the discharge side of the first-stage compression element 103c of the first compression mechanism 103, the second inlet-side intermediate branch tube 84 connected to the discharge side of the first-stage compression element 104c of the second compression mechanism 104, the intermediate header tube 82 whose flow merges with both inlet-side intermediate branch 25 tubes 81, 84, the first outlet-side intermediate branch tube 83 branching off from the intermediate header tube 82 and connected to the intake side of the second-stage compression element 103d of the first compression mechanism 103, and the second outlet-side intermediate branch tube 85 branching off from the intermediate header tube 82 and connected to the intake side of the second-stage compression element 104d of the second 30 compression mechanism 104. The discharge header tube 102b is a refrigerant tube for feeding refrigerant discharged from the compression mechanism 102 to the switching mechanism 3. A first oil separation mechanism 141 and a first non-return mechanism 142 are provided to the first discharge branch tube 103b connected to the discharge header tube 102b. A second oil separation mechanism 143 and a second non-return mechanism 144 are 55 provided to the second discharge branch tube 104b connected to the discharge header tube 102b. The first oil separation mechanism 141 is a mechanism whereby refrigeration oil that accompanies the refrigerant discharged from the first compression mechanism 103 is separated from the refrigerant and returned to the intake side of the compression mechanism 5 102. The first oil separation mechanism 141 mainly has a first oil separator 141a for separating from the refrigerant the refrigeration oil that accompanies the refrigerant discharged from the first compression mechanism 103, and a first oil return tube 141b that is connected to the first oil separator 141a and that is used for returning the refrigeration oil separated from the refrigerant to the intake side of the compression mechanism 102. The 10 second oil separation mechanism 143 is a mechanism whereby refrigeration oil that accompanies the refrigerant discharged from the second compression mechanism 104 is separated from the refrigerant and returned to the intake side of the compression mechanism 102. The second oil separation mechanism 143 mainly has a second oil separator 143a for separating from the refrigerant the refrigeration oil that accompanies the refrigerant 15 discharged from the second compression mechanism 104, and a second oil return tube 143b that is connected to the second oil separator 143a and that is used for returning the refrigeration oil separated from the refrigerant to the intake side of the compression mechanism 102. In the present modification, the first oil return tube 141b is connected to the second intake branch tube 104a, and the second oil return tube 143c is connected to the 20 first intake branch tube 103a. Accordingly, a greater amount of refrigeration oil returns to the compression mechanism 103, 104 that has the lesser amount of refrigeration oil even when there is an imbalance between the amount of refrigeration oil that accompanies the refrigerant discharged from the first compression mechanism 103 and the amount of refrigeration oil that accompanies the refrigerant discharged from the second compression 25 mechanism 104, which is due to the imbalance in the amount of refrigeration oil retained in the first compression mechanism 103 and the amount of refrigeration oil retained in the second compression mechanism 104. The imbalance between the amount of refrigeration oil retained in the first compression mechanism 103 and the amount of refrigeration oil retained in the second compression mechanism 104 is therefore resolved. In the present 30 modification, the first intake branch tube 103a is configured so that the portion leading from the flow juncture with the second oil return tube 143b to the flow juncture with the intake header tube 102a slopes downward toward the flow juncture with the intake header tube 102a, while the second intake branch tube 104a is configured so that the portion leading from the flow juncture with the first oil return tube 141b to the flow juncture with the intake header 56 tube 102a slopes downward toward the flow juncture with the intake header tube 102a. Therefore, even if either one of the compression mechanisms 103, 104 is stopped, refrigeration oil being returned from the oil return tube corresponding to the operating compression mechanism to the intake branch tube corresponding to the stopped compression 5 mechanism is returned to the intake header tube 102a, and there will be little likelihood of a shortage of oil supplied to the operating compression mechanism. The oil return tubes 141b, 143b are provided with pressure-reducing mechanisms 141c, 143c for depressurizing the refrigeration oil that flows through the oil return tubes 141b, 143b. The non-return mechanisms 142, 144 are mechanisms for allowing refrigerant to flow from the discharge 10 side of the compression mechanisms 103, 104 to the switching mechanism 3, and for blocking the flow of refrigerant from the switching mechanism 3 to the discharge side of the compression mechanisms 103, 104. Thus, in the present modification, the compression mechanism 102 is configured by connecting two compression mechanisms in parallel; namely, the first compression 15 mechanism 103 having two compression elements 103c, 103d and configured so that refrigerant discharged from the first-stage compression element of these compression elements 103c, 103d is sequentially compressed by the second-stage compression element, and the second compression mechanism 104 having two compression elements 104c, 104d and configured so that refrigerant discharged from the first-stage compression element of 20 these compression elements 104c, 104d is sequentially compressed by the second-stage compression element. In the present modification, the intermediate heat exchanger 7 is provided to the intermediate header tube 82 constituting the intermediate refrigerant tube 8, and the intermediate heat exchanger 7 is a heat exchanger for cooling the mixed flow of the 25 refrigerant discharged from the first-stage compression element 103c of the first compression mechanism 103 and the refrigerant discharged from the first-stage compression element 104c of the second compression mechanism 104. Specifically, the intermediate heat exchanger 7 functions as a shared cooler for two compression mechanisms 103, 104. Accordingly, the circuit configuration is simplified around the compression mechanism 102 when the 30 intermediate heat exchanger 7 is provided to the parallel-multistage-compression-type compression mechanism 102 in which a plurality of multistage-compression-type compression mechanisms 103, 104 are connected in parallel. The first inlet-side intermediate branch tube 81 constituting the intermediate refrigerant tube 8 is provided with a non-return mechanism 81a for allowing the flow of 57 refrigerant from the discharge side of the first-stage compression element 103c of the first compression mechanism 103 toward the intermediate header tube 82 and for blocking the flow of refrigerant from the intermediate header tube 82 toward the discharge side of the first-stage compression element 103c, while the second inlet-side intermediate branch tube 84 5 constituting the intermediate refrigerant tube 8 is provided with a non-retum mechanism 84a for allowing the flow of refrigerant from the discharge side of the first-stage compression element 104c of the second compression mechanism 104 toward the intermediate header tube 82 and for blocking the flow of refrigerant from the intermediate header tube 82 toward the discharge side of the first-stage compression element 104c. In the present modification, 10 non-return valves are used as the non-return mechanisms 81 a, 84a. Therefore, even if either one of the compression mechanisms 103, 104 has stopped, there are no instances in which refrigerant discharged from the first-stage compression element of the operating compression mechanism passes through the intermediate refrigerant tube 8 and travels to the discharge side of the first-stage compression element of the stopped compression mechanism. 15 Accordingly, there are no instances in which refrigerant discharged from the first-stage compression element of the operating compression mechanism passes through the interior of the first-stage compression element of the stopped compression mechanism and exits out through the intake side of the compression mechanism 102, which would cause the refrigeration oil of the stopped compression mechanism to flow out, and it is thus unlikely 20 that there will be insufficient refrigeration oil for starting up the stopped compression mechanism. In the case that the compression mechanisms 103, 104 are operated in order of priority (for example, in the case of a compression mechanism in which priority is given to operating the first compression mechanism 103), the stopped compression mechanism described above will always be the second compression mechanism 104, and therefore in this 25 case only the non-return mechanism 84a corresponding to the second compression mechanism 104 need be provided. In cases of a compression mechanism which prioritizes operating the first compression mechanism 103 as described above, since a shared intermediate refrigerant tube 8 is provided for both compression mechanisms 103, 104, the refrigerant discharged from the 30 first-stage compression element 103c corresponding to the operating first compression mechanism 103 passes through the second outlet-side intermediate branch tube 85 of the intermediate refrigerant tube 8 and travels to the intake side of the second-stage compression element 104d of the stopped second compression mechanism 104, whereby there is a danger that refrigerant discharged from the first-stage compression element 103c of the operating 58 first compression mechanism 103 will pass through the interior of the second-stage compression element 104d of the stopped second compression mechanism 104 and exit out through the discharge side of the compression mechanism 102, causing the refrigeration oil of the stopped second compression mechanism 104 to flow out, resulting in insufficient 5 refrigeration oil for starting up the stopped second compression mechanism 104. In view of this, an on/off valve 85a is provided to the second outlet-side intermediate branch tube 85 in the present modification, and when the second compression mechanism 104 has stopped, the flow of refrigerant through the second outlet-side intermediate branch tube 85 is blocked by the on/off valve 85a. The refrigerant discharged from the first-stage compression element 10 103c of the operating first compression mechanism 103 thereby no longer passes through the second outlet-side intermediate branch tube 85 of the intermediate refrigerant tube 8 and travels to the intake side of the second-stage compression element 104d of the stopped second compression mechanism 104; therefore, there are no longer any instances in which the refrigerant discharged from the first-stage compression element 103c of the operating first 15 compression mechanism 103 passes through the interior of the second-stage compression element 104d of the stopped second compression mechanism 104 and exits out through the discharge side of the compression mechanism 102 which causes the refrigeration oil of the stopped second compression mechanism 104 to flow out, and it is thereby even more unlikely that there will be insufficient refrigeration oil for starting up the stopped second compression 20 mechanism 104. An electromagnetic valve is used as the on/off valve 85a in the present modification. In the case of a compression mechanism which prioritizes operating the first compression mechanism 103, the second compression mechanism 104 is started up in continuation from the starting up of the first compression mechanism 103, but at this time, 25 since a shared intermediate refrigerant tube 8 is provided for both compression mechanisms 103, 104, the starting up takes place from a state in which the pressure in the discharge side of the first-stage compression element 103c of the second compression mechanism 104 and the pressure in the intake side of the second-stage compression element 103d are greater than the pressure in the intake side of the first-stage compression element 103c and the pressure in 30 the discharge side of the second-stage compression element 103d, and it is difficult to start up the second compression mechanism 104 in a stable manner. In view of this, in the present modification, there is provided a startup bypass tube 86 for connecting the discharge side of the first-stage compression element 104c of the second compression mechanism 104 and the intake side of the second-stage compression element 104d, and an on/off valve 86a is 59 provided to this startup bypass tube 86. In cases in which the second compression mechanism 104 has stopped, the flow of refrigerant through the startup bypass tube 86 is blocked by the on/off valve 86a and the flow of refrigerant through the second outlet-side intermediate branch tube 85 is blocked by the on/off valve 85a. When the second 5 compression mechanism 104 is started up, a state in which refrigerant is allowed to flow through the startup bypass tube 86 can be restored via the on/off valve 86a, whereby the refrigerant discharged from the first-stage compression element 104c of the second compression mechanism 104 is drawn into the second-stage compression element 104d via the startup bypass tube 86 without being mixed with the refrigerant discharged from the first 10 stage compression element 104c of the first compression mechanism 103, a state of allowing refrigerant to flow through the second outlet-side intermediate branch tube 85 can be restored via the on/off valve 85a at point in time when the operating state of the compression mechanism 102 has been stabilized (e.g., a point in time when the intake pressure, discharge pressure, and intermediate pressure of the compression mechanism 102 have been stabilized), 15 the flow of refrigerant through the startup bypass tube 86 can be blocked by the on/off valve 86a, and operation can transition to the normal air-cooling operation. In the present modification, one end of the startup bypass tube 86 is connected between the on/off valve 85a of the second outlet-side intermediate branch tube 85 and the intake side of the second-stage compression element 104d of the second compression mechanism 104, while the other end is 20 connected between the discharge side of the first-stage compression element 104c of the second compression mechanism 104 and the non-return mechanism 84a of the second inlet side intermediate branch tube 84, and when the second compression mechanism 104 is started up, the startup bypass tube 86 can be kept in a state of being substantially unaffected by the intermediate pressure portion of the first compression mechanism 103. An electromagnetic 25 valve is used as the on/off valve 86a in the present modification. The action of the air-conditioning apparatus I of the present modification during the air-cooling operation, the air-warming operation, and the like are essentially the same as the action in the above-described Modification 5 (FIGS. 22 through 24, 20, and 21 and the relevant descriptions), except that the points modified by the circuit configuration 30 surrounding the compression mechanism 102 are somewhat more complex due to the compression mechanism 102 being provided instead of the compression mechanism 2, for which reason the action is not described herein. The same operational effects as those of Modification 5 described above can also be achieved with the configuration of the present modification. 60 In the present modification, the switching between the air-cooling operation and air cooling start control, i.e., the switching between the refrigerant non-return state and the refrigerant return state is performed by the on/off states of the on/off valves 11, 12, 92a, but an intermediate heat exchanger switching valve 93 capable of switching between the 5 refrigerant non-return state and the refrigerant return state may be provided instead of the on/off valves 11, 12, 92a, as in Modification I described above. Furthermore, particularly advantageous effects can be obtained when using the configuration of the heat source unit I a such as the one in Modification 2. (9) Modification 7 10 In the above-described embodiment and the modifications thereof, either a two-stage compression-type compression mechanism 2 is configured such that the refrigerant discharged from the first-stage compression element is sequentially compressed in the second-stage compression element by one compressor 21 having a single-axis two-stage compression structure, or a two-stage compression-type compression mechanism 102 is 15 configured such that two single-axis two-stage compression-type compressors 29, 30 are connected in parallel, whereby the refrigerant discharged from the first-stage compression element is sequentially compressed by the second-stage compression element; but another option is to configure a two-stage compression-type compression mechanism such that compressors 22, 23 having single-stage compression structures are connected in series, 20 whereby the refrigerant discharged from the first-stage compression element is sequentially compressed by the second-stage compression element. For example, the refrigerant circuit 110 in Modification 1 described above (see FIG. 10) may be replaced by a refrigerant circuit 610 that uses a compression mechanism 202 in which compressors 22, 23 having single-stage compression structures are connected in series 25 instead of the compression mechanism 2 composed of a compressor 21 having a single-axis two-stage compression structure, as shown in FIG. 26. In the present modification, the compression mechanism 202 is configured from a compressor 22 in which refrigerant is compressed by a compression element 202c as a first stage compression element, and a compressor 23 in which refrigerant is compressed by a 30 compression element 202d as a second-stage compression element. The compressor 22 has a hermetic structure in which a casing 22a houses a compressor drive motor 22b, a drive shaft 22c, and a compression element 202c. The compressor drive motor 22b is linked to the drive shaft 22c. The compressor 23 has a hermetic structure in which a casing 23a houses a compressor drive motor 23b, a drive shaft 23c, and a compression element 202d. The 61 compressor drive motor 23b is linked to the drive shaft 23c. In the present modification, the compression elements 202c, 202d are rotary elements, scroll elements, or another type of positive displacement compression element. The compression mechanism 202 is configured so as to draw in refrigerant through an intake tube 2a, to discharge this refrigerant to an 5 intermediate refrigerant tube 8 after the refrigerant has been compressed by the compression element 202c of the compressor 22, to admit the refrigerant discharged to the intermediate refrigerant tube 8 into the compression element 202d of the compressor 23, and to discharge the refrigerant to a discharge tube 2b after the refrigerant has been further compressed. The action of the air-conditioning apparatus I of the present modification during the 10 air-cooling operation, the air-warming operation, and the like are essentially the same as the action in the above-described Modification I (FIGS. 10 and 1 through 9 and the relevant descriptions), except for the compression mechanism 2 being replaced by the compression mechanism 202, for which reason the action is not described herein. The same operational effects as those of Modification I and the like described above 15 can also be obtained with the configuration of the present modification. (10) Modification 8 In the embodiment described above and the modifications thereof, the intermediate heat exchanger return tube 94 is provided with the intermediate heat exchanger return on/off valve 94a composed of an electromagnetic valve, and control is performed for closing the 20 valve when the switching mechanism 3 is in the cooling operation state and opening the valve when the switching mechanism 3 is in the heating operation state, but instead of this intermediate heat exchanger return on/off valve 94a, another option is to provide a flow rate control valve so as to enable control of the quantity of refrigerant flowing through the intermediate heat exchanger 7 functioning as an evaporator of refrigerant during the air 25 warming operation. For example, the refrigerant circuit 610 in Modification 7 described above (see FIG. 26) may be replaced with a refrigerant circuit 710 provided with an intermediate heat exchanger return valve 94b as a flow rate control valve instead of the intermediate heat exchanger return on/off valve 94a, as shown in FIG. 27. In the present modification, an 30 electrically driven expansion valve whose opening degree can be adjusted is used as the intermediate heat exchanger return valve 94b. When the intermediate heat exchanger return valve 94b is provided, the first expansion mechanism 5a provided to the receiver inlet tube 18a is provided to a refrigerant tube 18h connecting the heat source-side heat exchanger 4 and the bridge circuit 17 together (more specifically, to the portion of the refrigerant tube 18h 62 between the branching position of the intermediate heat exchanger return tube 94 and the heat source-side heat exchanger 4), thereby ensuring a pressure difference in the area before the intermediate heat exchanger return valve 94b and the area after the intermediate heat exchanger return valve 94b. The second expansion mechanism 5b provided to the receiver 5 outlet tube 18b is also provided to a refrigerant tube 18i connecting the bridge circuit 17 and the usage-side heat exchanger 6 together, whereby the pressure of the refrigerant in the receiver 18 is brought to an intermediate pressure in the refrigeration cycle. The configuration of the present modification is different from that of Modification 7 described above in that refrigerant flows in the refrigerant circuit 710 sequentially through 10 the first expansion mechanism 5a, the receiver 18, and the second expansion mechanism 5b via the bridge circuit 17 during the air-cooling operation and refrigerant flows in the refrigerant circuit 710 sequentially through the second expansion mechanism 5b, the receiver 18, and the first expansion mechanism 5a via the bridge circuit 17 during the air-warming operation (in Modification 7, refrigerant flows in the refrigerant circuit 610 sequentially 15 through the first expansion mechanism 5a, the receiver, and the second expansion mechanism 5b during both the air-cooling operation and the air-warming operation), but otherwise the same operational effects as those of Modification 7 described above can be obtained. Moreover, in the configuration of the present modification, since the intermediate heat exchanger return valve 94b as a flow rate control valve is provided to the intermediate heat 20 exchanger return tube 94, not only can the flow of refrigerant into the intermediate heat exchanger return tube 94 be prevented during the air-cooling operation, but it is also possible to reliably distribute the flow rate of refrigerant flowing through the heat source-side heat exchanger 4 and the flow rate of refrigerant flowing through the intermediate heat exchanger 7 during the heating operation. 25 (11) Modification 9 In the configurations of the above-described embodiment and the modifications thereof, an expansion device for isentropically expanding the refrigerant flowing between the heat source-side heat exchanger 4 and the usage-side heat exchanger 6 may be provided between the heat source-side heat exchanger 4 and the usage-side heat exchanger 6. 30 For example, the refrigerant circuit 710 in Modification 8 described above (see FIG 27) may be replaced with a refrigerant circuit 810 in which an expansion device 97 for isentropically expanding the refrigerant is provided to the receiver inlet tube 18a, as shown in FIG 28. Specifically, in the present modification, the expansion device 97 is connected via the bridge circuit 17 as a rectifier circuit for rectifying the refrigerant flow so that the 63 refrigerant flows in from the inlet of the expansion device 97, both in cases in which the refrigerant flows from the heat source-side heat exchanger 4 to the usage-side heat exchanger 6 and cases in which the refrigerant flows from the usage-side heat exchanger 6 to the heat source-side heat exchanger 4. In the present modification, a centrifugal or positive 5 displacement expansion device is used as the expansion device 97. In the present modification, the bridge circuit 17 is used as a rectifier circuit, but the configuration may also be designed so that the same function is fulfilled by a four-way switching valve or by combining a plurality of electromagnetic valves. The same operational effects as those of Modification 8 and the like described above 10 can also be obtained with the configuration of the present modification. Moreover, in the configuration of the present modification, during the air-cooling operation, refrigerant flows in the refrigerant circuit 810 sequentially through the first expansion mechanism 5a, the expansion device 97, the receiver 18, and the second expansion mechanism 5b via the bridge circuit 17 as a rectifier circuit, and during the air-warming operation, refrigerant flows in the 15 refrigerant circuit 810 sequentially through the second expansion mechanism 5b, the receiver 18, and the first expansion mechanism 5a via the bridge circuit 17 as a rectifier circuit, whereby the refrigerant is isentropically depressurized by the expansion device 97 during the process in which the refrigerant is depressurized from a high pressure to a low pressure in the refrigeration cycle during both the air-cooling operation and the air-warming operation (in 20 other words, during the air-cooling operation, using FIGS. 3 and 4 as examples, the refrigerant is depressurized while point F moves to an area of lower enthalpy and lower entropy; and during the air-warming operation, using FIGS. 6 and 7 as examples, the refrigerant is depressurized while point E moves to an area of lower enthalpy and lower entropy). It is thereby possible to improve the coefficient of performance and to recover 25 energy, and operation efficiency during both the air-cooling operation and the air-warming operation can therefore be further improved. In the present modification, the depressurizing range in the expansion device 97 may be increased to maximize the improvement of operating efficiency, either by performing control for increasing the opening degree of the second expansion mechanism 5b downstream of the expansion device 97 and/or control for 30 opening the first intake return on/off valve 18g during the air-cooling operation, or by performing control for increasing the opening degree of the first expansion mechanism 5a downstream of the expansion device 97 and/or control for opening the first intake return on/off valve 18g during the air-cooling operation, for example. (12) Modification 10 64 In the configuration of Modification 9 described above, the receiver 18 positioned in the outlet of the expansion device 97 may be made to function as a gas-liquid separator, a second-stage injection tube may be connected for returning the gas refrigerant separated from the liquid in the receiver 18 to the second-stage compression element 2d, and intermediate 5 pressure injection may be performed by the receiver 18 as a gas-liquid separator during both the air-cooling operation and the air-warming operation. For example, the refrigerant circuit 810 (see FIG. 28) in Modification 9 described above may be replaced by a refrigerant circuit 910 in which the second second-stage injection tube 18c is connected to the receiver 18, and intermediate pressure injection can be 10 performed by the receiver 18 as a gas-liquid separator, as shown in FIG. 29. The second second-stage injection tube 18c is a refrigerant tube capable of performing intermediate pressure injection for removing refrigerant out of the receiver 18 and returning the refrigerant to the second-stage compression element 202d of the compression mechanism 202, and in the present modification, the second second-stage injection tube 18c 15 is provided so as to connect the top part of the receiver 18 with the intermediate refrigerant tube 8 (i.e., with the intake side of the second-stage compression element 202d of the compression mechanism 202). The second second-stage injection on/off valve 18d and the second second-stage injection non-return mechanism 18e are provided to the second second stage injection tube 18c. The second second-stage injection on/off valve 18d is a valve 20 capable of opening and closing, and is an electromagnetic valve in the present modification. The second second-stage injection non-return mechanism 18e is a mechanism for allowing the flow of refrigerant from the receiver 18 to the second-stage compression element 202d and for blocking the flow of refrigerant from the second-stage compression element 202d to the receiver 18, and a non-return valve is used in the present modification. The second 25 second-stage injection tube 18c and the first intake return tube 18f are integrated in the portion near the receiver 18. The same operational effects as those of Modification 9 described above can also be obtained with the configuration of the present modification. Moreover, in the configuration of the present modification, operation efficiency can be further improved because it is also 30 possible, during both the air-cooling operation and the air-warming operation, to cause the receiver 18 connected to the outlet of the expansion device 97 to function as a gas-liquid separator, to perform intermediate pressure injection for returning the gas refrigerant separated from the liquid in the receiver 18 to the second-stage compression element 202d through the second second-stage injection tube 18c (i.e., using FIGS. 20 and 21 as examples, 65 to perform a process for returning from point I to point G by way of point M), and thereby to reduce the temperature of the intermediate-pressure refrigerant in the refrigeration cycle drawn into the second-stage compression element 202d. (13) Modification 11 5 In Modifications 7 through 10 described above, the configuration may be designed having a plurality of usage-side heat exchangers 6 connected in parallel to each other, the objective being to perform air cooling or air warming according to the air-conditioning load of a plurality of air-conditioned spaces, for example. For example, the refrigerant circuits 810 and 910 in Modifications 9 and 10 10 described above (see FIGS. 28 and 29) may be replaced by refrigerant circuits 1010 and 1110 having a plurality (two in this case) of usage-side heat exchangers 6 connected to each other in parallel, as shown in FIGS. 30 and 31. When a plurality of usage-side heat exchangers 6 are provided, in order to control the flow rate of the refrigerant flowing through the usage side heat exchangers 6 and ensure that the refrigeration load required by the usage-side heat 15 exchangers 6 can be obtained, usage-side expansion mechanisms 5c are provided instead of the second expansion mechanism 5b between the receiver 18 and the usage-side heat exchangers 6, so as to correspond to the usage-side heat exchangers 6 (i.e., to portions in the refrigerant tube 18i branching off toward the usage-side heat exchangers 6). The same operational effects as those of Modifications 9 and 10 and the like 20 described above can also be obtained with the configuration of the present modification. (14) Modification 12 In Modifications 7 through 11 described above, a subcooler may be provided, the objective being to cool the refrigerant fed to the usage-side heat exchanger 6 and the heat source-side heat exchanger 4 so that the refrigerant reaches a subcooled state. 25 For example, the refrigerant circuit 1010 in Modification 11 described above (see FIG 30) may be replaced by a refrigerant circuit 1210, as shown in FIG. 32, in which a subcooling heat exchanger 96 is provided to the receiver outlet tube 18b and a third intake return tube 95 is provided to the portion extending through the receiver 18 from the receiver inlet tube 18a to the receiver outlet tube 18b (i.e., is provided to the receiver 18). 30 The subcooling heat exchanger 96 is a heat exchanger for cooling refrigerant fed from the receiver 18 through the plurality (two in this case) of usage-side expansion mechanisms 5c to the usage-side heat exchangers 6 during the air-cooling operation, and also for cooling refrigerant fed from the receiver 18 through the first expansion mechanism 5a and the intermediate heat exchanger return valve 94b to the heat source-side heat exchanger 4 and 66 the intermediate heat exchanger 7 during the air-warming operation. More specifically, the subcooling heat exchanger 96 is a heat exchanger for performing heat exchange with the refrigerant flowing through the third intake return tube 95 which returns from the receiver 18 to the intake side of the compression mechanism 2 (i.e., to the intake tube 2a). The third 5 intake return tube 95 is provided with a third intake return valve 95a whose opening degree can be controlled, and during the air-cooling operation in the subcooling heat exchanger 96, heat exchange is performed between the refrigerant fed from the receiver 18 to the usage-side expansion mechanisms 5c and the refrigerant flowing through the third intake return tube 95 after being depressurized to a nearly low pressure in the third intake return valve 95a, and 10 heat exchange is also performed between the refrigerant fed from the receiver 18 to the first expansion mechanism 5a and the intermediate heat exchanger return valve 94b and the refrigerant flowing through the third intake return tube 95 after being depressurized to a nearly low pressure in the third intake return valve 95a. The third intake return valve 95a is an electrically driven expansion valve in the present modification. The third intake return 15 tube 95 and the first intake return tube 18f are integrated in the portion near the receiver 18. The same operational effects as those of Modification 11 and the like described above can also be obtained with the configuration of the present modification. Moreover, in the configuration of the present modification, the refrigerant fed from the receiver 18 to the usage-side expansion mechanisms 5c can be brought to a subcooled state during the air 20 cooling operation, and the refrigerant fed from the receiver 18 to the first expansion mechanism 5a and the intermediate heat exchanger return valve 94b can be brought to a subcooled state during the air-warming operation (in other words, using FIGS. 23 and 24 as examples, the process from point I to point R is performed). Therefore, it is thereby possible to reduce the risk of an imbalanced flow of refrigerant being distributed to the usage 25 side expansion mechanisms 5c during the air-cooling operation, and also to reduce the risk of an imbalanced flow of refrigerant being distributed to the first expansion mechanism 5a and the intermediate heat exchanger return valve 94b during the air-warming operation. (15) Modification 13 In the above-described embodiment and the modifications thereof, two-stage 30 compression-type compression mechanisms 2, 102, 202 are used, but three-stage compression systems or compression mechanisms having even more stages may also be used. For example, in the refrigerant circuit 1010 in Modification 11 described above (see FIG. 30), a three-stage compression-type compression mechanism 302 may be used in which single-stage-compression compressors 25, 26, 27 identical to the compressors 22, 23 67 constituting the compression mechanism 202 are connected in series; the intermediate refrigerant tube 8 for connecting the discharge of the first compressor 25 and the intake of the second compressor 26 may be provided with the same intermediate heat exchanger 7, intermediate heat exchanger bypass tube 9, second intake return tube 92, intermediate heat 5 exchanger switching valve 93, and intermediate heat exchanger return tube 94 as those of the above-described embodiment and modifications thereof; and an intermediate refrigerant tube 308 for connecting the intake of the second compressor 26 with the third compressor 27 may be provided with an intermediate heat exchanger 307, an intermediate heat exchanger bypass tube 309, a second intake return tube 392, an intermediate heat exchanger switching valve 10 393, and an intermediate heat exchanger return tube 394 identical to the intermediate heat exchanger 7, the intermediate heat exchanger bypass tube 9, the second intake return tube 92, the intermediate heat exchanger switching valve 93, and the intermediate heat exchanger return tube 94, as shown in FIG. 33. The configuration of the present modification differs from that of the above 15 described Modification 11, for example, in that since the three-stage compression-type compression mechanism 302 is used, the intermediate heat exchangers 7, 307 can be made to function as coolers of the intermediate-pressure refrigerant in the refrigeration cycle (the refrigerant fed to the second-stage compression element 302d after being discharged from the first-stage compression element 302c, and the refrigerant fed to the second-stage compression 20 element 302e after being discharged from the first-stage compression element 303c) by switching the intermediate heat exchanger switching valves 93, 393 to the refrigerant non return state during the air-cooling operation, and the intermediate heat exchangers 7, 307 can be made to function as evaporators of the low-pressure refrigerant in the refrigeration cycle (the refrigerant whose heat is radiated in the usage-side heat exchangers 6) by switching the 25 intermediate heat exchanger switching valves 93, 393 to the refrigerant return state during the air-warming operation. However, aside from this difference, it is possible to obtain the same operational effects as those of the above-described Modification 11, for example. (16) Other embodiments Embodiments of the present invention and modifications thereof are described above 30 with reference to the drawings, but the specific configuration is not limited to these embodiments or their modifications, and can be changed within a range that does not deviate from the scope of the invention. For example, in the above-described embodiment and modifications thereof, the present invention may be applied to a "chiller-type" air-conditioning apparatus in which water 68 or brine is used as a heating source or cooling source for conducting heat exchange with the refrigerant flowing through the usage-side heat exchanger 6, and a secondary heat exchanger is provided for conducting heat exchange between indoor air and the water or brine that has undergone heat exchange in the usage-side heat exchanger 6. 5 The present invention can also be applied to other types of refrigeration apparatuses besides the above-described chiller-type air-conditioning apparatus, as long as the apparatus performs a multistage compression refrigeration cycle using a refrigerant that operates in a supercritical range as its refrigerant. The refrigerant that operates in a supercritical range is not limited to carbon dioxide; 10 ethylene, ethane, nitric oxide, and other gases may also be used. INDUSTRIAL APPLICABILITY If the present invention is used, high operation efficiency is obtained in a refrigeration apparatus which has a refrigerant circuit configured to be capable of switching between a cooling operation and a heating operation and which performs a multistage 15 compression-type refrigeration cycle. 69