METHOD FOR CONTROLLING HELIOSTAT USED FOR CONDENSING OF SUNLIGHT AND DEVICE THEREOF TECHNICAL FIELD (0001] The present invention relates to a method for controlling a heliostat used for collecting of sunlight for the purpose of tracking the sun and collecting reflected light at an arbitrary point (a focal point), and relates to a device of the method. BACKGROUND ART [0002] In recent years, the depletion and the soaring prices of petroleum resources have been matters of concern. In the meantime, a shift from petroleum resources, which come to be one of the causes of global warming, to alternative energy resources has been studied. As one of such alternative energy sources, solar thermal power generation is available in which sunlight is collected and used as energy. [0003] In the solar thermal power generation, a heliostat used for collecting of sunlight includes multiple reflecting mirrors (facets) . The heliostat is either configured so that sunlight can be reflected and collected at a heat receiving part or the like and then the heat thereof is used for electric power generation, or configured as a center-reflector-type solar thermal power generation plant in which light beams reflected from facets are re-reflected from a large reflecting mirror (a center reflector) and collected at a heat receiving part. Here, for the purpose of increasing a power generation efficiency, proposed is the invention in which a heliostat is configured to track movements of sunlight (refer to Patent Document 1, for 1 example) [0004] Fig. 14 illustrates a lateral view of an example of a heliostat used for solar thermal power generation. A conventional heliostat 5 includes multiple facets 20. Multiple sets (3 sets in Fig. 14) each including the facet 20 disposed on a pedestal 47 are provided on a rotation mechanism 45. The thus provided facets 20 respectively have seats connected to each other by a link mechanism 46. Accordingly, it is configured that the link mechanism 46 causes the conventional heliostat 5 to perform rise-and-fall motions 44, and that the rotation mechanism 45 causes the conventional heliostat 5 to perform rotation motions 43. It is configured that these motions enable the conventional heliostat 5 to track the sun and to reflect and collect the sunlight at an arbitrary place (for example, a heat receiving part, a reflecting mirror, or the like in the solar thermal power generation). [0005] Fig. 15 is a schematic plan view illustrating the appearance of the facets 20 mounted on the conventional heliostat 5. In general, multiple facets 20 are installed in combination of a certain number thereof (14 facets in Fig. 15) . The facets 20 described here each have one side of approximately 450 mm. PRIOR ART DOCUMENT PATENT DOCUMENT [0006] Patent Document 1: Japanese patent application Kokai publication No. 2004-37037 SUMMARY OF THE INVENTION 2 PROBLEM TO BE SOLVED BY THE INVENTION [0007] However, a heliostat described in Patent Document 1 is configured so as to track sunlight by rotation on the X-axis and the Y-axis as shown in Fig. 3 of Patent Document 1. The facets move around an intersection point of the X-axis and the Y-axis of the heliostat. Accordingly, a phenomenon (comatic aberration) occurs in which the position of a focal point formed by reflected light beams from the respective facets deviates, leading to a problem of low light collecting rate. The same applies to the above-described conventional heliostat 5 shown in Fig. 14. The conventional heliostat 5 also has a problem in which a focal point distance of the facet 20 located away from the center deviates due to the rotation 43 of the conventional heliostat 5 around the center of the rotation mechanism 45 as a base point. [0008] This phenomenon of focal point deviation (comatic aberration) will be described with reference to Fig. 8 and Fig. 9. Fig. 8 is a pattern diagram of the multiple facets 20 (3 facets in Fig. 8) being installed in the heliostat 5, and shows how the heliostat operates around a rise-and-fall and rotation center 0 as a base point. [0009] The facets 20 are installed with their angles adjusted in advance so that the facets 20 can reflect sunlight S irradiated from the sun 40 and thus reflected light beams R can form a focal point F at, for example, a heat receiving part, a reflecting mirror, or the like. Fig. 9 shows a situation when the sun 40 has moved. The movement of the sun 40 changes the 3 angle of the sunlight S irradiated on the respective facets 20. Accordingly, the heliostat 5 again performs rotation motions as well as rise-and-fall motions in order to correct the position of the focal point for light collecting. [0010] At this time, rotation motions or rise-and-fall motions of the heliostat 2 are performed around the above-described rise-and-fall and rotation center 0 as a base point. Accordingly, the facet 20 located on the left in Fig. 9 ends up shifting upward in the drawing by a facet shift distance d, while the facet 20 located on the right as well ends up shifting downward in the drawing by the facet shift distance d. Therefore, the reflected light beams R do not form a focal point at a position which should be the focal point F on a heat receiving part or the like as shown in Fig. 9, resulting in the situation where the reflected light beams R diffuse by a shift distance e from the focal point. This phenomenon is called comatic aberration. Even if the heliostat 5 is adjusted in its installation so that the reflected light beams R intersect with each other at the focal point F, the reflected light beams R would not intersect with each other at the focal point F because of the rotation and rise-and-fall motions. (0011] The comatic aberration described above results in a decrease in light collecting efficiency. Thus, there arises a problem especially for a solar thermal power generation plant, which uses a large-scale number of the heliostats as many as hundreds or thousands, that a decrease in light collecting efficiency causes a significant decrease in power generation efficiency of the plant. 4 [0012] Thus, the present invention has been made in order to solve the above-described problems. An object of the present invention is to provide a method for controlling a heliostat used for collecting of sunlight and a device of the method, the method and the device achieving: a high sunlight collecting efficiency by allowing no deviation of a light collecting point (a focal point) of sunlight; and a high area arrangement efficiency by having a configuration in which no rotation by means of a rotation mechanism is carried out. [00131 Additionally, an object of the present invention is to reduce costs for installation work of a device in a solar thermal power generation plant by having a device configuration which allows the installation and adjustment work of facets to be carried out easily, and is further to provide a highly-efficient solar thermal power generation plant. MEANS FOR SOLVING THE PROBLEM [0014] A method for controlling a heliostat according to the present invention to achieve the above object is a method for controlling a heliostat, which is used for collecting of sunlight and has multiple reflecting mirrors, in a way that the heliostat tracks the sun in motion, reflects sunlight, and collects the sunlight at a predetermined focal point. The method is characterized as follows. Specifically, the method includes the steps of: adjusting the multiple reflecting mirrors so that the multiple reflecting mirrors have a focal point at a predetermined distance; and controlling the multiple reflecting mirrors so that the multiple reflecting mirrors 5 track the sun and light beams reflected from the respective reflecting mirrors have a focal point at an arbitrary point, the multiple reflecting mirrors being configured to tilt in conjunction with each other. The reflecting mirrors are operated in conjunction with each other while maintaining a state where coordinates of predetermined points of the respective reflecting mirrors are fixed. [0015] In this configuration, control is designed such that the multiple facets each have a center for rise-and-fall motions and rotation motions (tilt motions); therefore, it is possible to prevent occurrence of the comatic aberration. Specifically, by having a configuration in which each of the facets 20 has a rise-and-fall and rotation center as shown in Fig. 10, the control method allows the facet shift distance d to be zero, thereby preventing occurrence of the comatic aberration. [0016] Further, control is designed such that the multiple facets track the sun in conjunction with each other. Accordingly, after the multiple facets are adjusted in the early stage so as to have a focal point at an arbitrary position, the focal point can be easily maintained. [0017] Here, in solar thermal power generation, control is carried out in such a way that the position of a focal point formed by reflected light beams is kept constant regardless of movements of the sun (a light source) . The principle in this control is the same as that of the above-described control to move the focal point. [0018] 6 The above-described method for controlling a heliostat is characterized in that the reflecting mirrors are operated in conjunction with each other while maintaining a state where coordinates of centers of the respective reflecting mirrors are fixed. [0019] In this configuration, the center of each facet is set as a center for rise-and-fall motions and rotation motions (tilt motions); therefore, even the comatic aberration occurring at an end portion of the facet can be prevented. In the present invention, the facet supposedly has a size of 450 square mm to 1000 square mm. If, for example, the rise-and-fall and rotation center of a facet is set at a corner of the facet, the distance from the rise-and-fall and rotation center to the other edge corner would be large, resulting in a shift distance d of the facet. (0020] In this respect, the center of the facet is set as the rise-and-fall and rotation center in this control method. Accordingly, the facet shift distance d can be brought as close as possible to zero; therefore, occurrence of the comatic aberration can be inhibited within a significantly small range. [0021] The above-described method for controlling a heliostat is characterized in that a focal point formed by the multiple reflecting mirrors is movable on a celestial sphere surface having an arbitrary radius without occurrence of comatic aberration. [00221 The above-described method for controlling a heliostat 7 is characterized in that directions of the multiple reflecting mirrors are controlled in conjunction with each other by two different link mechanisms. [0023] In this configuration, control on the facets for guiding the reflected light beams to an arbitrary direction is carried out by a link mechanism, which has at least two different vector directions, simultaneously on the multiple facets. Therefore, positional control on the facets can be easily and reliably achieved with a simple mechanism. [0024] A heliostat to achieve the above object is a heliostat used for collecting of sunlight and configured to arrange multiple reflecting mirrors therein in such a way that the multiple reflecting mirrors have a focal point. The heliostat is characterized as follows. Specifically, the multiple reflecting mirrors are mounted on pedestals with tilting mechanisms in between, respectively. The multiple tilting mechanisms are connected to each other by a first axis link (X-axis link) and a second axis link (Y-axis link) which are directed in two different directions. The multiple tilting mechanisms change an orientation in conjunction with each other by the links. [0025] In this configuration, the multiple tilting mechanisms are connected to each other with two shafts, which are the first axis link and the second axis link, directed in different directions. Accordingly, the heliostat can be easily controlled while maintaining the focal point of the reflected light beams by tilting the multiple facets simultaneously. 8 [00261 The above-described heliostat is characterized as follows. Specifically, the first axis link (X-axis link) and the second axis link (Y-axis link) are rod-shaped links, are provided in directions orthogonal to each other, and are connected to respective driving devices. The driving devices are controlled so as to configure that a position of a focal point of the multiple reflecting mirrors is movable through the respective links and the respective tilting mechanisms. [0027] In this configuration, the multiple tilting mechanisms are connected to each other with the first axis link (X-axis link) and the second axis link (Y-axis link) which are orthogonal to each other. Accordingly, the relationship between the amount of operation of the above driving device and the shift distance of the focal point can be easily calculated. Therefore, the control on the facets itself can be easily carried out. Further, the use of the rod-shaped links can secure a large motion space of the facets. This allows tracking of movements of the sun in a wide range, especially in a large-scale solar thermal power generation plant, leading to an improved power generation efficiency. [0028] A solar thermal power generation plant to achieve the above object is characterized as follows. Specifically, the plant includes multiple heliostats described above, and sunlight is collected at a heat receiving part using a molten salt as a heat medium so as to carry out solar thermal power generation. [0029] 9 In this configuration, the area efficiency of the heliostats provided in the solar thermal power generation plant is improved while the reflected light beams can be collected at a heat receiving part, a reflecting mirror, or the like. Accordingly, a solar thermal power generation plant having a significantly high power generation ef ficiency can be provided. Further, the multiple facets are tilted by means of a two-shaft link mechanism. Accordingly, transportation of materials to a site where a solar thermal power generation plant is to be installed as well as the installation can be easily carried out. Therefore, installation costs of the power generation plant can be reduced. EFFECTS OF THE INVENTION [00301 The method for controlling a heliostat and the device of the method according to the present invention can provide a method for controlling a heliostat used for collecting of sunlight and a device of the method, which achieve: a high sunlight collecting efficiency by allowing no deviation of a light collecting point (a focal point) of sunlight; and a high area arrangement efficiency by having a configuration in which no rotation by means of a rotation mechanism is carried out. [0031] Additionally, it is also possible to reduce costs for installation work of the device in a solar thermal power generation plant by having a device configuration which allows the installation and adjustment work of facets to be carried out easily, and further possible to provide a highly-efficient solar thermal power generation plant. BRIEF DESCRIPTION OF THE DRAWINGS 10 [00321 [Fig. 1] Fig. lis a partial enlarged view of a first example of the present invention. [Fig. 2] Fig. 2 is a schematic view of the first example of the present invention. [Fig. 3] Fig. 3 is a lateral view of a second example of the present invention. [Fig. 4] Fig. 4 is a lateral view of the second example 6f the present invention. [Fig. 5] Fig. 5 is a lateral view of a third example of the present invention. [Fig. 6] Fig. 6 is a schematic view illustrating how the third example of the present invention operates. [Fig. 7] Fig. 7 is a lateral view of a fourth example of the present invention. [Fig. 8] Fig. 8 is a schematic view illustrating the relationship between sunlight and its reflected light beams in a conventional heliostat. [Fig. 9] Fig. 9 is a schematic view illustrating occurrence of comatic aberration in the conventional heliostat. [Fig. 10] Fig. 10 is a schematic view illustrating the relationship between sunlight and its reflected light beams in a heliostat of the present invention. [Fig. 11] Fig. 11 is a schematic view illustrating a movement locus of a focal point in the heliostat of the present invention. [Fig. 12] Fig. 12 is a schematic view of a solar thermal power generation plant using the heliostat of the present invention. [Fig. 13] Fig. 13 is a schematic view of a solar thermal power generation plant using the conventional heliostat. [Fig. 14] Fig. 14 is a schematic view illustrating the 11 conventional heliostat. [Fig. 15] Fig. 15is a schematic plan view illustrating facets arranged in the conventional heliostat. MODES FOR CARRYING OUT THE INVENTION [0033] Hereinafter, the present invention will be described specifically by referring to embodiments illustrated in the drawings. EXAMPLE 1 [0034] Fig. 1 shows a partial enlarged view of a heliostat 1A which is a first example of the present invention. Fig. 2 shows a perspective view of the heliostat 1A including nine facets 20. The facets 20 are fixed by facet bolts 19 to tilting mechanisms 10A, respectively. The tilting mechanisms 10A are disposed on pedestals 16A, respectively. Further, the tilting mechanisms 16A are configured to move in conjunction with each other by being connected to each other in an X-axis direction by an X-axis link 11A with X-axis arm parts 13 in between, and being connected to each other in a Y-axis direction by a Y-axis link 12A with universal joints 15 and cylinder mechanisms 14A in between. Here, installation angles of the facets 20 are adjusted by the facet bolts 19 in advance so that a focal point can be formed at an arbitrary point. Fig. 2 shows an example of a case where multiple facets 20 are combined to form the heliostat 1A. Here, nine facets 20 are connected to each other in the X-axis direction and the Y-axis direction by the links. Edge portions of the links are connected to an X-axis driving device 17 and a Y-axis driving device 18, respectively. 12 It is configured that a link mechanism is moved by operating the driving devices 17 and 18 so that inclinations of the facets 20 can be controlled with two shafts. The facets 20 are adjusted in advance so that a focal point can be formed at an arbitrary point. When all the facets 20 are moved simultaneously by means of the link mechanism from this state, only the position of the focal point can be moved while reflected light beams are being focused on the point. Accordingly, in a solar thermal power generation plant, for example, reflected light beams are always focused on a heat receiving part, a reflecting mirror, or the like even when the sun moves. Therefore, it is possible to provide a plant having a significantly high power generation efficiency with no occurrence of comatic aberration or with minimal comatic aberration. In addition, by having a configuration of the link mechanism as shown in Fig. 2, a large motion space for the facets 20 can be secured. Accordingly, in a solar thermal power generation plant, it is possible to increase the range of being able to track the sun, thereby improving the power generating efficiency. Further, by changing the shape of the tilting mechanism 10A, it is possible to achieve a configuration in which the facets 20 can be tilted almost at 90 degrees in all directions. Especially in a huge solar thermal power generation plant having a size of hundreds square meters or larger, it is necessary to tilt the facets 20 largely. When the motion space of the facets 20 is increased, it is possible to lower the position where a heat receiving part, a reflecting mirror, or the like is disposed. Thus, it is possible to achieve cost reduction in constructing the solar thermal power 13 generation plant. EXAMPLE 2 [0035] Fig. 3 shows a schematic front view of a heliostat 1B which is a second example of the present invention. Fig. 4 shows a schematic lateral view thereof. The heliostat 1B is configured in such a way that facets 20 each having on a lower side thereof a tilting mechanism 10B rotate in horizontal directions shown in Fig. 3 around a Y-axis link 12B. The multiple tilting mechanism lOB are connected to each other by an X-axis link 11B as a link mechanism, and are configured to connect the multiple facets 20 aligned in a horizontal direction (the X-axis direction) in Fig. 3 so as to cause the multiple facets 20 to rise and fall in conjunction with each other. Further, rise-and-fall motions in the Y-axis directions (front-and-back directions with respect to the sheet of Fig. 3, or horizontal directions in Fig. 4), which are perpendicular to the X-axis directions in Fig. 3, can be achieved by the links connecting to the Y-axis links 12B through the multiple facets 20 as shown in Fig. 4. The present example allows formation of a compact link mechanism, thereby being capable of reducing the size of the structure of the heliostat 1B itself. Thus, costs of manufacturing and transporting the heliostat 1B can be reduced. EXAMPLE 3 [0036] Fig. 5 shows an outline of a heliostat 3A which is a third example of the present invention. Fig. 6 shows how the heliostat 3A tracks sunlight. The heliostat 3A includes multiple facets 20 each having in a lower portion thereof a 14 columnar supporting member 36. The multiple facets 20 are aligned so as to have a focal point. The supporting member 36 is formed of a flexible cylinder mechanism 34, and has a neck portion composed of a spherical joint. The neck portion is rotatably supported by an intermediate fixing plate 32 with a rotation mechanism 31 in between. The rotation mechanism 31 at the neck portion may be obtained by a joint having a degree of freedom equal to 2 other than the aforementioned spherical joint. Upper portions of the supporting member 36 are connected to the facets 20, respectively, with an installation angle adjustment mechanism 30 in between. When the heliostat is installed, the installation angles of the respective facets 20 are adjusted by the installation angle adjustment mechanism 30 so that reflected light beams from the respective multiple facets 20 can have a focal point at anarbitrary distance. Lower portions of the respective supporting members 36 are connected to each other by a link mechanism 35. When the link mechanism 35 moves on a plane surface, inclinations of the respective multiple facets 20 can be adjusted in conjunction with each other. Further, the link mechanism 35 moves on so-called an X-Y axis surface on a plane surface. For this reason, the connection between the supporting members 36 and the link mechanism 35 uses a joint which is operable in two X- and Y-axes, and desirably uses a spherical joint. As shown in Fig. 6, when the link mechanism 35 moves on an upper surface of a bottom plate 33, the facets 20 can each change the mirror-surface direction, as apparent from the directions of the respective facet normals. The link mechanism 35 can be moved by having the cylinder mechanisms 34 to extend. 15 Further, in the case of installation in a solar thermal power generation plant or the like, the link mechanism 35 is, controlled when the facets 20 track the sun, so that the sunlight can be always collected at the focal point on a heat receiving part, a reflecting mirror, or the like. Having the above-described configuration, the heliostat 3A appears as a heliostat having two layers including the bottom plate 33 and the intermediate fixing plate 32, and having a grove of the supporting members 36 extending below the facets 20. In addition, the facets 20 stick out from the intermediate fixing plate 32 as seen like head parts. EXAMPLE 4 [0037] Fig. 7 shows a schematic view of.a heliostat 3B which is a fourth example of the present invention. This heliostat 3B is an example when non-flexible supporting members 36 are used instead of the flexible cylinder mechanism in the third example. The supporting members 36 have at neck portions thereof rotation mechanisms 31 supported by an intermediate fixing plate 32. Accordingly, when the supporting members 36 having no flexibility are used, the link mechanism 35 moves in the three-dimensional space as if floating from the bottom plate 33. By having a configuration using no cylinder mechanism 34, the structure of the heliostat 3B can be simplified. Accordingly, when, for example, a solar thermal power generation plant is constructed in a desert, risks of a breakdown and the like due to sand and heat can be reduced. It is extremely important to use heliostats requiring less maintenance in a solar thermal power generation plant which uses 16 hundreds or thousands of the heliostat 3B. In other words, since costs for power generation are largely affected by the amount of maintenance required, the costs for power generation can be reduced by the present example. [0038) (Effects of carrying out the present invention) Fig. 10 is a schematic view illustrating the appearances of sunlight S and reflected light beams R in the heliostats 1A and 1B to which the controlling method and the device using the controlling method of the present invention are applied. Each of the facets 20 has an own rise-and-fall and rotation center o of the facet 20. Accordingly, even when the facets 20 moves as tracking the sun 40, a deviation (a shift distance e from the focal point) of the reflected light beams R from a focal point F as shown in Fig. 9 does not occur. Especially in solar thermal power generation plants, the distances from the focal point to the facets 20 are hundreds of meters to thousands of meters in some cases, depending on the scale of the plant. In such cases, even if the shift distance d of the facets is small, the shift distance e from the focal point will be huge. For this reason, with the method for controlling a heliostat and the device of the method according to the present invention, which allow occurrence of no comatic aberration (e~0), it is now possible to provide a highly efficient solar thermal power generation plant. Fig. 11 schematically shows a movement locus of the focal point F in the state where no comatic aberration occurs. When the position of the focal point F is moved by tilting the facets 20, the focal point Fmoves on a celestial sphere 41 with a focal point distance constant. This shows the state of zero comatic 17 aberration. It should be noted that, a solar thermal power generation plant is configured that the reflected light beams R are always collected at a heat receiving part, a reflecting mirror (a center reflector), or the like, that is, the focal point F is being fixed, while the sun as a light source is being tracked. This also is similarly affected by the comatic aberration. Accordingly, utilization of the present invention enables that the reflected light beams R are collected at a fixed position regardless of movements of the sun without being affected by the comatic aberration. Thus, a method for controlling a heliostat and a device of the method which achieve a high sunlight collecting efficiency can be provided. [0039] (Laying in solar thermal power generation plant) Fig. 13 shows a schematic view of a solar thermal power generation plant 6 in which conventional heliostats 5 are provided. Being rotated by means of a rotation mechanism 45 as shown in Fig. 14, the conventional heliostats 5 need to be disposed so that heliostat rotation ranges 42 shown in Fig. 13 may not overlap each other. On the other hand, having no conventional rotation mechanism, the heliostats 1A and 1B of the present invention can be arranged at smaller intervals from adjacent ones, thereby achieving a high area arrangement efficiency. Specifically, the number of heliostats which can be mounted for a heat receiving part or a center reflector arranged at the focal point F can be largely increased; thus, it has become possible to achieve significant improvement in the power generation efficiency in the solar thermal power generation plant 2. 18 [0040] As described above, according to the present invention, a method for controlling a heliostat used for collecting of sunlight and a device of the method can be provided, the method and the device achieving: a high sunlight collecting efficiency by allowing no deviation of a light collecting point (a focal point) of sunlight; and a high area arrangement efficiency by having a configuration in which no rotation by means of a rotation mechanism is carried out. [0041] Further, a reduction in costs for installation work of the device in a solar thermal power generation plant is achieved by having a device configuration which allows the installation and adjustment work of facets to be carried out easily. In addition, a highly-efficient solar thermal power generation plant can be provided. EXPLANATION OF REFERENCE NUMERALS [0042] 1A, 1B X-Y driven heliostat 2 solar thermal power generation plant 3A, 3B X-Y driven heliostat 10 tilting mechanism 11 X-axis link 12 Y-axis link 13 X-axis arm part 14 cylinder mechanism 15 universal joint 16 pedestal 17 X-axis driving device 18 Y-axis driving device 19 19 facet bolt 20 facet (reflecting mirror) 20