DESCRIPTION TITLE OF THE INVENTION: FRESHWATER PRODUCTION APPARATUS AND METHOD FOR PRODUCING 5 FRESHWATER TECHNICAL FIELD [0001] The present invention relates to a fresh water production apparatus and a 0 method for producing fresh water, in which raw water is treated with a pre-treatment device to produce pre-treated water and the pre-treated water is treated with a reverse osmosis membrane device to produce fresh water. BACKGROUND ART 5 [0002] Fresh water production apparatus including a reverse osmosis membrane module, which are used for seawater desalination, etc., are coming to be increasingly used in various fields because the apparatus have the merits of energy saving and space saving. Examples thereof include application to a water purification process in which 0 industrial water or tap water is produced from river water, ground water, or treated wastewater and application to a reverse osmosis membrane treatment step for sewage or wastewater recycling or to a reverse osmosis membrane treatment step for seawater desalination. The reverse osmosis membrane separator for use in seawater desalination or the like is basically configured so that a liquid to be treated is 5 pressurized to a given pressure with a high pressure pump and is fed to reverse osmosis membrane modules and dissolved components in the liquid to be treated are separated 1 by the reverse osmotic function of the reverse osmosis membrane module to obtain permeate. [0003] In a fresh water production apparatus including a reverse osmosis 5 membrane module, raw water is sent from a raw water tank 1 to a pre-treatment device 5 by a raw water feed pump 3 and is filtrated, as shown in Fig. 5. In the pre-treatment step, a pre-treatment device such as a sand filtration device or a pre-treatment membrane device with a microfiltration membrane or an ultrafiltration membrane has generally been used conventionally. The pre-treated water obtained through filtration 10 in the pre-treatment device 5 has been temporarily stored in an intermediate tank 15 and the pre-treated water has been fed to the reverse osmosis membrane module 9 of a reverse osmosis membrane device 10 using a high pressure pump 7, followed by treating. However, this method has a drawback that microorganisms grow in the intermediate tank 15 to render the reverse osmosis membrane module 9 susceptible to 15 fouling. In addition, there are cases where due to the installation of the intermediate tank 15, the high pressure pump 7 becomes insufficient in suction pressure. In such cases, it is necessary to dispose another booster pump 16 on the feed side of the high pressure pump 7, resulting in the necessity of an additional equipment cost. [0004] . 20 In order to solve this problem, a means has been proposed in which the pre treated water is directly fed to the reverse osmosis membrane device by the pre treatment device, followed by treating, as described, for example, in Patent Documents 1 and 2. A feature of this means resides in that it is not necessary to dispose the intermediate tank which has hitherto been necessary. 25 [0005] 2 Meanwhile, filtration with the pre-treatment device, regardless of whether the device is a sand filtration device or a pre-treatment membrane device, results in the accumulation therein of substances which are contained in the raw water and are to be removed, such as suspended substances, organic substances, and inorganic substances, 5 and this accumulation heightens the filtration resistance of the pre-treatment device and shortly renders the filtration unable to be continued. Consequently, for diminishing the increase in the filtration resistance of the pre-treatment device, it is necessary to periodically conduct backwashing in which water is caused to flow reversely from the pre-treated water side to the raw water side in the pre-treatment device. (Hereinafter, 10 referred to as a backwashing step.) During this backwashing step, either one of the plurality of disposed pre-treatment step lines or limited lines of the pre-treatment device are separated from the operation lines. In the case where there is no intermediate tank, the pre-treatment device and the reverse osmosis membrane device have been directly connected to each other and, hence, there is a possibility that when the backwashing 15 step of the pre-treatment device is initiated or when the pre-treatment device is shifted to an filtration step, pressure fluctuations might be given to the reverse osmosis membrane device to impair the stable operation of the reverse osmosis membrane device. [00061 20 Although Patent Document 1 proposes a means in which concentrated water from reverse osmosis membranes is used as backwashing water for pre-treatment membranes, any specific method for operation during the backwashing step is not described therein and there is a possibility that the reverse osmosis membrane device cannot be stably operated. Patent Document 2 proposes a means in which, in a system 25 including a pre-treatment device and a reverse osmosis membrane device directly connected thereto, liquid feeding to the reverse osmosis membrane device is stably 3 conducted even when certain lines of the pre-treatment device have shifted to a backwashing step, by using feed flow rate control for the pre-treatment device and flow rate control for the reverse osmosis membrane device. However, there is a possibility that the serial application of flow rate control to such lines where the pre-treatment 5 device and the reverse osmosis membrane device have been directly connected to each other might result in interference between the individual flow rate control operations and result in divergence of the flow rate control. BACKGROUND ART DOCUMENT 10 PATENT DOCUMENT [0007] Patent Document 1: JP-A-10-263539 Patent Document 2: JP-A-2007-181822 .5 SUMMARY OF THE INVENTION PROBLEMS THAT THE INVENTION IS TO SOLVE [0008] The present invention provides: a fresh water production apparatus in which pre-treated water obtained by pre-treating raw water with a pre-treatment device such as 0 a sand filtration device or a pre-treatment membrane module including a microfiltration membrane and/or an ultrafiltration membrane, is directly fed to the reverse osmosis membrane device without via an intermediate tank, in which backwashing of the pre treatment device can be performed without impairing the stable operation of the reverse osmosis membrane device; and a method for producing fresh water from raw water by 5 the fresh water production apparatus. 4 MEANS FOR SOLVING THE PROBLEMS [0009] In order to solve the problems described above, the fresh water production apparatus and method for producing fresh water of the invention have the following 5 features. (1) A fresh water production apparatus including: a pre-treatment device including a plurality of lines for treating raw water to produce pre-treated water; a reverse osmosis membrane device for treating the pre-treated water to 10 produce fresh water; a raw water feed pump for feeding the raw water to the pre-treatment device; a high pressure pump for feeding the pre-treated water to the reverse osmosis membrane device; 5 a first raw water feed pipeline in which one end thereof is connected to a raw water feed unit and the other end thereof is connected to the raw water feed pump; a second raw water feed pipeline in which one end thereof is connected to the raw water feed pump and the other end thereof is connected to each of the lines of the pre-treatment device; 0 a first connecting pipeline in which one end thereof is connected to the each of the lines of the pre-treatment device and the other end thereof is connected to the high pressure pump; and a second connecting pipeline in which one end thereof is connected to the high pressure pump and the other end thereof is connected to the reverse osmosis 5 membrane device, 5 in which the apparatus further includes: a pressure gauge disposed on the first connecting pipeline for measuring a treated-water pressure of the pre-treated water to be fed to the high pressure pump; a bypass pipeline which branches off on the second raw water feed pipeline for discharging the raw water from the system; and a pressure 5 control valve disposed on the bypass pipeline for controlling the treated-water pressure. (2) The fresh water production apparatus according to (1), which includes, on the second raw water feed pipeline, a flow meter for measuring a flow rate of the raw water to be fed to the each of the lines of the pre-treatment device and a flow control valve for controlling the flow rate of the raw water to be fed to the each of the lines of 10 the pre-treatment device. (3) A method for producing fresh water from raw water by the fresh water production apparatus according to (1) or (2), the method including controlling the treated-water pressure so as to keep constant using the pressure gauge and the pressure control valve. 15 (4) A method for producing fresh water from raw water by the fresh water production apparatus according to (2), in which the method comprises a backwashing step of the pre-treatment device, and a flow rate of the raw water to be fed to the pre treatment device during the backwashing step is controlled so as to keep constant using the flow meter and the flow control valve. 20 (5) The method for producing fresh water according to (4), which includes controlling the treated-water pressure so as to keep constant using the pressure gauge and the pressure control valve. ADVANTAGE OF THE INVENTION [0015] 6 According to the invention, backwashing of the pre-treatment device can be performed by a simple method while preventing the reverse osmosis membrane from being fouled, and the reverse osmosis membrane device can be stably operated. 5 BRIEF DESCRIPTION OF THE DRAWINGS [0016] [Fig. 1] Fig. 1 is a schematic apparatus flow chart showing one example of the fresh water production apparatus of the invention. [Fig, 2] Fig. 2 is a schematic view showing one example of the pre-treatment device .0 including a membrane module according to the invention. [Fig. 3] Fig. 3 is a schematic apparatus flow chart showing another example of the fresh water production apparatus of the invention. [Fig. 4] Fig. 4 is a schematic view showing another example of the pre-treatment device including a membrane module according to the invention. 5 [Fig. 5] Fig. 5 is a schematic apparatus flow chart showing one example of conventional fresh water production apparatus. MODE FOR CARRYING OUT THE INVENTION [0017] 0 The present invention will be explained below in more detail on the basis of the embodiments shown in the drawings. Incidentally, the invention should not be construed as being limited to the following embodiments. [0018] The fresh water production apparatus of the invention includes, for example 5 as shown in Fig, 1, a raw water tank 1 for storing raw water therein; a pre-treatment device 5 including a plurality of lines for treating the raw water to produce pre-treated 7 water; a reverse osmosis membrane device 10 for treating the pre-treated water to produce fresh water; a raw water feed pump 3 for feeding the raw water to the pre treatment device 5; a high pressure pump 7 for feeding the pre-treated water to the reverse osmosis membrane device 10; a first raw water feed pipeline 2 in which one end 5 thereof is connected to the raw water tank 1 and the other end thereof is connected to the raw water feed pump 3; a second raw water feed pipeline 4 in which one end thereof is connected to the raw water feed pump 3 and the other end thereof is connected to each of the lines of the pre-treatment device 5; a first connecting pipeline 6 in which one end thereof is connected to the each of the lines of the pre-treatment device 5 and 10 the other end thereof is connected to the high pressure pump 7; and a second connecting pipeline 8 in which one end thereof is connected to the high pressure pump 7 and the other end thereof is connected to the reverse osmosis membrane device 10. The fresh water production apparatus further includes a pressure gauge 11 disposed on the first connecting pipeline 6 for measuring the treated-water pressure of the pre-treated water .5 to be fed to the high pressure pump 7; a bypass pipeline 12 which branches off on the second raw water feed pipeline 4 for discharging the raw water from the system; and a pressure control valve 13 disposed on the bypass pipeline 12 for controlling the treated water pressure. [0019] 0 Examples of raw water to which the invention is applicable include various kinds of raw water such as seawater, river water, ground water, and treated wastewater. [0020] The pre-treatment device 5 includes a plurality of lines so as to accommodate a backwashing step, and the device 5 may be a sand filtration device or 5 may have a structure in which sand filtration devices are directly connected to each 8 other in a two-stage arrangement. Alternatively, the device 5 may be a pre-treatment device including a microfiltration membrane and/or an ultrafiltration membrane. [0021] A pre-treatment membrane module 20 to be used in the pre-treatment device 5 5 is not particularly limited so long as it is a microfiltration membrane capable of rejecting particles or polymers having a size of 0.1 jtm or larger or an ultrafiltration membrane capable of rejecting particles or polymers having a size of 2 nm or larger but smaller than 0.1 pm. With respect to the type of the microfiltration membrane and/or ultrafiltration membrane for use in the pre-treatment membrane modules 20, a hollow 10 fiber membrane type, flat sheet membrane type, spiral type, or tubular type may be used, but the hollow fiber membrane type is preferred from the standpoint of cost reduction. With respect to the mode of membrane filtration, either dead end filtration type module or cross flow filtration type module may be used, but the dead end filtration type module is preferred from the standpoint that this type is low in energy 15 consumption. Furthermore, although the pre-treatment membrane module may be either pressurized type module or submerged type module, pressurized type module is preferred from the standpoint that this type is capable of high-flux operation. Moreover, the pre-treatment membrane module may be either of the external-pressure type in which raw water is fed from outside of the membrane and permeate is obtained 20 from inside or of the internal-pressure type in which raw water is fed from inside of the membrane and permeate is obtained from outside. However, the external-pressure type is preferred from the standpoint of the simplicity of the pre-treatment. [0022] The material of the microfiltration membrane and/or ultrafiltration 5 membrane to be used in the pre-treatment membrane module 20 is not particularly limited, and examples thereof include polysulfone, polyethersulfone, polyacrylonitrile, 9 polyimide, polyetherimide, polyamide, polyetherketone, polyetheretherketone, polyethylene, polypropylene, ethylene/vinyl alcohol copolymer, cellulose, cellulose acetate, poly(vinylidene fluoride), ethylene/tetrafluoroethylene copolymer, polytetrafluoroethylene, and composite materials thereof Of these, poly(vinylidene 5 fluoride) is preferred as the material of the microfiltration membrane and/or ultrafiltration membrane because poly(vinylidene fluoride) has excellent chemical resistance and, hence, periodic washing of the microfiltration membrane and/or ultrafiltration membrane with a chemical enables the microfiltration membrane and/or ultrafiltration membrane to recover the filtration function, leading to a prolongation of 10 the life of the pre-treatment membrane module. [00231 As the material of the case of the pre-treatment membrane module 20, the following may be used either alone or as a mixture: polyolefin such as polyethylene, polypropylene, and polybutene, fluororesin such as polytetrafluoroethylene (PTFE), 5 tetrafluoroethylene/perfluoroalkyl vinyl ether copolymers (PFA), fluoroethylene/polypropylene copolymers (FEP), ethylene/tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), trifluorochloroethylene/ethylene copolymer (ECTFE), and poly(vinylidene fluoride) (PVDF), chlororesin such as poly(vinyl chloride) and poly(vinylidene chloride), and 0 other resins including polysulfone resin, polyethersulfone resin, polyallylsulfone resin, poly(phenyl ether) resin, acrylonitrile/butadiene/styrene copolymer resin (ABS), acrylonitrile/styrene copolymer resin, poly(phenylene sulfide) resin, polyamide resin, polycarbonate resin, polyetherketone resin, and polyetheretherketone resin. Preferred materials other than resins are aluminum, stainless steel, and the like. Furthermore, a 5 composite material such as a resin/metal composite, a glass fiber-reinforced resin, or a carbon fiber-reinforced resin may be used. 10 [0024] The reverse osmosis membrane module 9 in the reverse osmosis membrane device 10 according to the invention is configured by disposing one element or a plurality of serially connected elements in a pressure vessel, the element(s) being a 5 spiral element obtained by winding flat sheet membranes around a collection tube, a plate-and-frame type element produced by applying flat sheet membranes to both surfaces of a plate type support to obtain a membrane-covered support and stacking such membrane-covered supports with spacers interposed therebetween so as to leave a given space therebetween to form a module, a tubular element in which tubular 10 membranes are used, or a hollow fiber membrane element obtained by bundling hollow fiber membranes and disposing the bundle in a case. Although the type of the elements may be any of these types, it is preferred to use spiral elements from the standpoints of operating efficiency and interchangeability. The number of elements can be set arbitrarily in accordance with the membrane perfonnance. In the case of 5 using spiral elements, the number of elements to be packed into one module is preferably such that one to about eight elements are serially arranged therein. A plurality of reverse osmosis membrane modules 9 may be disposed in parallel. [0025] The reverse osmosis membrane which constitutes the reverse osmosis 0 membrane module 9 is not particularly limited, and any reverse osmosis membrane having desalting performance may be used. However, as the material thereof, for example, a polyamide-based material, a polypiperazine-amide-based material, a polyester-amide-based material, or a material obtained by crosslinking a water-soluble vinyl polymer can be used. With respect to membrane structure, a membrane having a 5 dense layer in at least one surface of the membrane and having micropores, the pore diameter of which gradually becomes larger from the dense layer toward an inner part 11 of the membrane or toward the membrane on the other surface (asymmetric membrane), a membrane which has an exceedingly thin separatory layer formed on the dense layer of such an asymmetric membrane and made of another material (composite membrane), or the like may be used. However, it is preferred that the reverse osmosis membrane 5 should be composite membrane from the standpoint of high water production amount. From the standpoints of water permeation amount, chemical resistance, etc., polyamide based composite membrane is preferred of those, and piperazine-polyamide-based composite membrane is more preferred. [0026] 10 The material of each of the first raw water feed pipeline 2, second raw water feed pipeline 4, first connecting pipeline 6, second connecting pipeline 8, and bypass pipeline 12 may be any of resinous pipelines such as PVC pipes and polyethylene pipes, metallic pipes such as carbon steel pipes and stainless steel pipes, and lining pipes obtained by lining the inner surfaces of metallic pipes with a resin. It is, however, 5 necessary to select pipes while taking account of the quality of the raw water and the pressure necessary for the apparatus. In particular, since the second connecting pipeline 8 is frequently exposed to a high pressure, it is preferred to use not a resinous pipeline but a metallic pipe or a lining pipe. [0027] 0 As the raw water feed pump 3, a centrifugal pump is generally used. With respect to the high pressure pump 7, a centrifugal pump or a plunger pump is generally used. [0028] Examples of the raw water feed unit include a raw water tank 1. With 5 respect to the material of the raw water tank 1, the raw water tank I may be either a concrete tank or a resinous tank made of polyethylene, polypropylene, FRP, or the like. 12 In the case where the raw water to be treated is directly introduced from sea, river, a water well, or the like, there are cases where the raw water tank 1 is not disposed. [0029] In order to explain the backwashing step within the pre-treatment device 5 5 in the invention, an example of schematic views of the pre-treatment device 5 including a membrane module is shown in Fig. 2. The pre-treatment device 5 includes a pre treatment membrane module 20, a raw water feed valve 21 provided to the second raw water feed pipeline 4, a filtration valve 22 provided to the first connecting pipeline 6 on the pre-treated water side, a backwashing valve 23 provided to a backwashing pipeline 10 26, a backwashing drain valve 24 provided to a backwashing discharge pipeline 27, and a backwashing pump 25 for feeding backwashing water through the backwashing pipeline 26. The pre-treatment device 5 may further include, besides those, pipelines/valves for air scrubbing, pipelines/valves for air discharge, and the like. The sand filtration device has a similar structure. In the invention, however, the pre 15 treatment device 5 using a membrane module is explained below as a model. [0030] In the invention, the pre-treatment device 5 and the reverse osmosis membrane device 10 are directly connected to each other without via an intermediate tank or the like, and the pre-treatment device 5 periodically shifts from a filtration step 0 to a backwashing step and is separated from the operation lines. Because of this, similar pre-treatment membrane modules 20 for constituting a plurality of lines are necessary although not shown in Fig. 2. [0031] The backwashing step which is periodically conducted in order to inhibit the 5 pre-treatment membrane modules 20 from increasing in filtration resistance is as follows. Of the multiple pre-treatment membrane modules 20 independently disposed 13 in parallel, some pre-treatment membrane modules 20 are kept being operated to filtrate raw water, and the remaining pre-treatment membrane modules 20, which are desired to be subjected to a backwashing step, are each operated in the following manner. The raw water feed valve 21 and the filtration valve 22 are closed to stop the filtration, and 5 the backwashing valve 23 and the backwashing drain valve 24 are opened. Then, the pre-treated water stored in a pre-treatment membrane water tank is fed to the pre-treated water side of the pre-treatment membrane module by the backwashing pump 25. As the backwashing water, the pre-treated water, etc. may be used. The backwashing water which has passed through the pre-treatment membranes in the direction reverse to 10 the filtration direction passes through the backwashing drain valve 24 which is opened, and is discharged as washing drainage from the pre-treatment membrane module 20. Thus, the backwashing step is performed. After the backwashing is conducted for a given period, the backwashing pump 25 is stopped and the backwashing valve 23 is closed. 5 [0032] Simultaneously with or subsequently to this backwashing, an air scrubbing step can be conducted in which pressurized air is fed to a lower part of the pre-treatment membrane module 20 to wash the pre-treatment membranes so as to oscillate the pre treatment membranes, although this step is not shown in the figure. 0 [0033] After the backwashing, a drain valve, not shown, which is provided to the lower part of the pre-treatment membrane module 20 is opened, and the washing drainage held on the raw water side in the pre-treatment membrane module 20 are thereby discharged from the per-treatment membrane module 20. 5 [0034] 14 A flow control valve 32 and the raw water feed valve 21 are opened to thereby feed raw water to the pre-treatment membrane module 20 which has undergone the backwashing, and the air that remains on the raw water side in the pre-treatment membrane module 20 goes out through the backwashing drain valve 24 which is 5 opened. In this step, a flushing step in which the washing drainage held on the raw water side in the pre-treatment membrane module 20 are discharged through the backwashing drain valve 24, can be applied. In this case, discharge of the washing drainage through the drain valve may be conducted or not. After the air has completely gone out through the backwashing drain valve 24, the backwashing drain 0 valve 24 is closed and the filtration valve 22 is opened. Thus, the backwashing step of the pre-treatment membrane module 20 is terminated, and the pretreatment membrane module 20 which has been undergoing the backwashing step returns to the filtration step like the other pre-treatment membrane modules 20. [0035] 5 In the invention, the pre-treatment device 5 includes a plurality of lines such as that shown in Fig. 2, and the lines are each independently and periodically subjected repeatedly to an operation in which the line shifts from a filtration step to a backwashing step and returns to the filtration step after completion of the backwashing step, as described above. When such step shifts occur, the number of operation lines 0 increases or decreases and, hence, pressure fluctuations within the system occur undesirably. In case where the pressure fluctuations are large, the decreases in supply pressure for the high pressure pump 7 causes interlock to thereby shut down the reverse osmosis membrane device 10 and exert a considerable influence on the flow control of the reverse osmosis membrane device 10. Consequently, to minimize the pressure 5 fluctuations is a preferred condition for operating the pre-treatment device 5 and the reverse osmosis membrane device 10 directly connected thereto. 15 [0036] Accordingly, the method for producing fresh water from raw water by the fresh water production apparatus of the invention is characterized in that when the backwashing step of the pre-treatment device 5 is conducted, the treated-water pressure 5 of the pre-treated water that is measured by the pressure gauge 11 disposed on the first connecting pipeline 6, is controlled using the pressure control valve 13 disposed on the bypass pipeline 12 which branches off on the second raw water feed pipeline 4 for discharging the raw water from the system, by a pressure control unit 14 for controlling the reverse osmosis membrane supply pressure, as shown in Fig. 1. l0 [0037] In the case where some lines of the pre-treatment device 5 including a plurality of lines have shifted from a filtration step to a backwashing step, the pressure for feeding the pre-treated water to the high pressure pump 7 decreases due to the resultant decrease in the number of operation lines. However, in the fresh water 5 production apparatus of the invention and in the method for producing fresh water of the invention, the pressure control valve 13 is automatically turned in the closing direction by the pressure control unit 14 to keep the internal pressure of the system constant. Conversely, in the case where those lines of the per-treatment device 5 which have been in the backwashing step return to the filtration step, the pressure for 0 feeding the pre-treated water to the high pressure pump 7 increases due to the resultant increase in the number of operation lines. In the fresh water production apparatus of the invention, however, the pressure control valve 13 is automatically turned in the opening direction by the pressure control unit 14 to keep the internal pressure of the system constant. 5 [0038] 16 It is preferred that the fresh water production apparatus of the invention, besides having the configuration of a fresh water production apparatus shown in Fig. I described above, includes, on the second raw water feed pipeline 4, a flow meter 31 for measuring the flow rate of the raw water to be fed to the each of the lines of the pre 5 treatment device 5 and a flow control valve 32 for controlling the flow rate of the raw water to be fed to the each of the lines of the pre-treatment device 5, as shown, for example, in Fig. 3. [0039] In the case where the pre-treatment device 5 includes membrane modules, 10 raw water is fed in the final stage of the backwashing step to discharge the air remaining on the raw water side of the pre-treatment membrane module 20 through the backwashing drain valve 24 as described above. Also in the case where the pre treatment device 5 is a sand filtration device, it is necessary to conduct raw water feeding, as a rinse step, in the final stage of the backwashing step to discharge the 5 remaining air through the backwashing drain valve, etc. over a given period after the backwashing in order to stabilize the filter medium. [0040] In either case, raw water is fed through the second raw water feed pipeline 4 which is connected also to the other lines that are being continuously operated and in 0 which a given pressure is maintained, and the raw water is discharged, in the pre treatment device 5 in the state of undergoing the backwashing step, from the system through the backwashing drain valve 24 and the backwashing discharge pipeline 27 to the discharge water side which is opened to the atmosphere. Because of this, raw water temporarily flows in a large amount into the pre-treatment device 5 which is 5 undergoing the backwashing step, and the internal pressure of the other lines which are being continuously operated, is considerably reduced. 17 [0041] The fresh water production method for producing fresh water from raw water by the fresh water production apparatus of the invention is hence characterized by the following: the configuration shown in Fig. 3 described above is used and, in the case 5 where it is necessary to feed raw water to the pre-treatment device 5 in the state of undergoing a backwashing step, the flow rate of the raw water to be fed to the each of the lines of the pre-treatment device 5, measured by the flow meter 31 disposed for the each of the lines of the pre-treatment device 5, are controlled using the flow control valve 32 for controlling the flow rate of the raw water to be fed to the each of the lines 10 of the pre-treatment device 5, by a flow control unit 33 for controlling the flow rate of the raw water to be fed to the pre-treatment device 5. [0042] By controlling the raw water feed rate during a backwashing step as described above, the raw water can be fed to the pre-treatment device 5 which is 5 undergoing the backwashing step, without lowering the pressure of the raw water feed pipelines 4 of the other lines that are being continuously operated. [0043] Incidentally, the raw water flow rate control described above in the pre treatment device 5 during a backwashing step using the flow meter 31 and the flow 0 control valve 32 can be used for raw water flow rate control in the pre-treatment device 5 in which a filtration step is being conducted. In general, however, flow rate control of the pre-treated water to be fed to the reverse osmosis membrane device 10 which is the stage subsequent to the pre-treatment device 5, is also conducted. In this case, the flow rate control operations are serially connected, resulting in a possibility that the 5 control might diverge due to the interference effect of the individual control operations. It is therefore preferred that the raw water flow rate control in the pre-treatment device 18 5 according to the invention is used only for the raw water feeding during the backwashing step and that during the filtration step, the flow rate control is not conducted and the flow control valve 32 is fully opened. [0044] 5 It is preferred that the pressure gauge 11 is an electronic pressure transmitter. The material of the liquid-contact portion thereof is selected while taking account of the quality of the raw water, like the material of the first connecting pipeline 6. [0045] 10 The pressure control valve 13 and the flow control valve 32 each may be either an electrically operated valve or a pneumatically operated valve. However, the pneumatically operated valve equipped with a positioner is preferred when responsiveness is taken into account. The valve main body generally is globe valve, butterfly valve, or the like. .5 [0046] The pressure control unit 14 and the flow control unit 33 are generally operated by PID control, and may be controlled either by using the PID control function of a PLC or DCS for controlling the whole fresh water production apparatus or by using a newly disposed PID one-loop controller. 0 [0047] For diminishing pressure fluctuations when the pre-treatment device 5 undergoes a step shift, it is desirable to open/close the raw water feed valve 21 which takes part in the step shift, and the filtration valve 22 which is disposed on the pre treated water side, as slowly as possible. For this purpose, it is preferred to dispose a 5 speed controller or a positioner in the case where the raw water feed valve 21 and the filtration valve 22 are pneumatic valves. 19 [0048] Thus, the supply pressure for the reverse osmosis membrane device 10 can be always kept constant. Consequently, the reverse osmosis membrane device 10 can be stably operated without stopping the reverse osmosis membrane device 10 or without 5 affecting the flow rate control of the reverse osmosis membrane device 10. [0049] While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and 10 scope thereof. This application is based on Japanese patent applications No. 2011-201349 filed on September 15, 2011 and No. 2012-058395 filed on March 15, 2012, the contents of which are incorporated herein by reference. 15 DESCRIPTION OF REFERENCE NUMERALS AND SIGNS [0050] 1: Raw water tank 2: First raw water feed pipeline 3: Raw water feed pump 20 4: Second raw water feed pipeline 5: Pre-treatment device 6: First connecting pipeline 7: High pressure pump 8: Second connecting pipeline 25 9: Reverse osmosis membrane module 10: Reverse osmosis membrane device 20 11: Pressure gauge 12: Bypass pipeline 13: Pressure control valve 14: Pressure control unit 5 15: Intermediate tank 16: Booster pump 20: Pre-treatment membrane module 21: Raw water feed valve 22: Filtration valve 10 23: Backwashing valve 24: Backwashing drain valve 25: Backwashing pump 26: Backwashing pipeline 27: Backwashing discharge pipeline 15 31: Flow meter 32: Flow control valve 33: Flow control unit 21