HURRY DAM AND METHOD OF APPLICATION OF THE SAME BACKGROUND OF THE INVENTION Field of the Invention The present invention refers to a rubber dam, which crosses a landfill of a dam (movable) or a river and dam water and a method of application of it. Description of the Related Art As shown in Figure 8, an arch dam 30 as a dam, which damped the current of a river and adjusts the amount of running water, has been presented for practical use. This arch dam 30 is applied in the following manner. A sheet of rubber 32 bends and covers to form a bag shape. The sheet of rubber in the form of a bag 32 is placed on a landfill dam 14, which is constructed in a river bed. A seal portion 32A of the bag-shaped rubber sheet 32 is fixed to the upper portion of the weir dam 14 by bolts 20, so that the rubber sheet 32 is formed in an arc shape. When the rubber sheet 32 is bent into an arc shape, due to the difference in curvature, the rubber sheet 32 is tensioned in the longitudinal direction on the fixed side thereof (on the upstream side). Accordingly, when the rubber sheet 32 is folded, a force by which the rubber sheet 32 contracts to the upstream side, acts on the rubber sheet 32 and as shown in Figure 9, the Rubber sheet 32 is bent into portions on the downstream side. In this way, if the rubber sheet 32 is buckled, when the rubber sheet 32 is folded, it can be struck by a floating wood or the like and can cause damage or abrasions. When two end portions of the rubber sheet 32 are fixed to the side surfaces of the fixed holds in this manner since the rubber sheet 32 buckles along the sloping surface of the landfill dam 14, the field application is inferior. SUMMARY OF THE INVENTION In view of the aforementioned facts, it is an object of the present invention to provide a rubber dam, in which a rubber sheet does not buckle in portions during the contraction (folding) of the same and which has good field of application. The first aspect of the present invention is a rubber dam that adjusts the amount of running water, comprising: a landfill dam that is constructed in a riverbed; a rubber sheet formed in a bag shape; a fixing portion, in which the rubber sheet in the form of a bag is fixed to the highest portion of the landfill dam; and supports holding the rubber sheet in the form of a bag when folded, extending substantially horizontally from the highest portion of the landfill dam to the downstream side, at a predetermined distance from each other in the direction of width of the river. The second aspect of the present invention is a method for applying a rubber dam that adjusts the amount of running water; which includes the stages of: building a landfill dam on a riverbed; build a landfill dam on a riverbed; constructing supports supporting a sheet of rubber in the form of a pouch, which has been folded, extending substantially horizontally from the highest portion of the landfill dam to the downstream side, at a predetermined distance from each other in the direction across the river; and placing the sheet of rubber in the form of a bag in the supports and fixing the sheet of rubber in the form of a bag to the highest portion of the weir dam. In the present invention, the rubber sheet is formed in a bag shape and a seal portion of the rubber sheet in the form of a bag can be attached to the highest portion of the landfill dam. In this way, when the leaf-shaped rubber sheet expands, the rubber dam rises, damming water and adjusting the amount of running water. When the sheet of sheet rubber contracts and folds, the rubber sheet is held by the supports, which extend substantially horizontally from the highest portion of the landfill dam to the downstream side at a predetermined distance from each other in the widthwise direction of the river. For this reason, the rubber sheet hangs with its own weight and with the weight of the water that runs downstream in order to hang between the supports. According to the above, the rubber sheet on the downstream side does not buckle. Furthermore, since the rubber sheet can be placed in the supports and two end portions of it can be fixed to the lateral surfaces of the fixed dams. As a result, the field of application can be improved. Since the present invention is structured as described above, when the rubber sheet is folded, the rubber sheet does not buckle on the downstream side. According to the above, the rubber sheet is not damaged by floating wood or the like. In addition, when the rubber sheet is folded, because the rubber sheet does not contact the landfill dam or the like, abrasion does not occur between the rubber sheet and the landfill dam. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view illustrating a state, in which a rubber dam has been raised according to an embodiment of the present invention. Figure 2 is a plan view illustrating a state, in which the rubber dam has been raised according to the present embodiment. Figure 3 is a cross-sectional view illustrating a state, in which the rubber dam has been folded according to present modality. Figure 4 is a perspective view illustrating a state, in which the rubber dam has been folded according to the present embodiment. Figure 5 is a front view illustrating a state, in which the rubber dam has been folded according to the present embodiment, as seen from the downstream side. Figure 6 is a front view illustrating a state, in which a straight-type rubber dam has been folded, as seen from the downstream side. Figure 7 is a perspective view illustrating a variant example of supports. Figure 8 is a perspective view illustrating a state, in which a conventional rubber dam has been raised. Figure 9 is a perspective view illustrating a state, in which a conventional arch rubber dam has been folded. DESCRIPTION OF THE PREFERRED EMBODIMENTS In Figures 1 to 5, an arch dam 10 according to an embodiment of the present invention is shown. The arch dam 10 is structured in such a way that a strip-shaped rubber sheet 12 is bent and a coated joint portion 12A is fixed to a higher portion 14A of an arch-shaped weir dam 14, which is made of concrete and is built on a riverbed. At this point, a bias cord or a reinforcement cord can be embedded in the rubber sheet. The bolt holes are formed in the joint portion 12A. These bolt holes are pressed by a plate 18 and the gasket portion 12A of the rubber sheet 12 is fixed to the upper portion 14A of the landfill dam 14, when using the bolts 20. In addition, two end portions of the rubber sheet 12 (on the sides of the edges) are fixed to the side surfaces of the fixed holds 50 to form a bag shape. A supply / discharge pipe (not shown) for communicating with an inner portion of the rubber sheet
12 is placed in the landfill dam 14. The fluid is supplied to the arch dam 10 or discharged therefrom by its supply / discharge pipe in order to raise or fold the arch dam 10 A plurality of pipes 40 as a support extends horizontally from the highest portion 14A of the landfill dam 14 to the center of the curvature of the arch, at a fixed distance from each other in the widthwise direction of the river. The distal ends of these pipes 40 are supported by columns 42 which are laid straight with respect to the river bed, such that a predetermined support resistance can be maintained. A method of applying the rubber dam in accordance with the present embodiment and an operation thereof will be explained hereafter. First, after the landfill dam 14 has been constructed, the columns 42 are placed straight in the river bed. The pipes 40 connect areas between the columns 42 and the highest portion 14A. Then, the bag-shaped rubber sheet 12 is placed in the pipes 40. The seal portion 12A of the rubber sheet 12 is fixed to the upper portion 14A of the landfill dam 14 by the bolts 20 and the two end portions of the rubber sheet 12 are fixed to the side surfaces of the fixed holds 50. In this way since the rubber sheet 12 is placed in the pipes 40, when the two end portions of the rubber sheet 12 can be fixed to the lateral surfaces of the fixed dams, the field of application of the arch dam can be improved. In such a structure as described above, when the sheet of bag-shaped rubber 12 which has expanded shrinks and collapses, as shown in Figures 4 and 5, the rubber sheet 12 hangs so as to fall down between the pipes 40 in a wave pattern (ie, the so-called dragon wave) along the widthwise direction of the river. According to the above, the rubber sheet 12 does not buckle on the downstream side and can thus absorb with inertia. In the present embodiment, the height of the columns 42 or the diameter or pitch of the pipes 40 can be designed according to the amount of overflow water or the width of the river. In addition, it is possible to reduce the flow resistance acting with respect to the rubber sheet 12 by placing the pipes 40 at an inclination in. Go down to the downstream side. In the present embodiment, an arch rubber dam has been explained. However, obviously, the present invention can be applied to a straight rubber dam. In a case of such a straight-type rubber dam, as shown in Figure 6, in providing the pipes 40, even when the rubber sheet 54 contracts, the rubber sheet 54 does not sink downwardly as indicated by the dotted line and instead forms such a wave pattern as shown by the solid line. According to the above, when the overflow occurs, it is possible to reduce the water pressure applied to the two end portions of the rubber sheet 54. In the present embodiment, the pipes 40 have been used as supports. However, the present invention is not limited thereto. As shown in Figure 7, it is possible to put concrete at an inclination of the landfill dam 14 to form wall bodies 52 provided on the downstream side whose upper portions are substantially parallel with the surface of the water. In addition, a rubber dam to which the present invention can be applied is not limited to the rubber sheet that is bent. Instead, a rubber dam that is formed into a bag shape can be used by connecting two end portions of the rubber sheets to each other, respectively.