The present invention relates to a storage phosphor plate and, more particularly, to a storage phosphor plate for use in a radiation image recording and reproducing method, and to a radiation image storage panel employed in the method. With the recent spread of the CT scan, there has been desired a system which can record the radiation image of a human body and which is capable of reproducing the recorded radiation image at any time and will not deteriorate over time. For that purpose, there has been developed a system for recording and reproducing the radiation image of a human body based on a stimulative photoradiation or a so-called stimulative luminescence phenomenon wherein the radiation energy of an electromagnetic wave such as X-rays, xcex1-rays, xcex3-rays, electron beams or ultraviolet radiation absorbed in a certain substance is stored and then is emitted in the form of light by impingement of the stimulating light, as described, for example, in U.S. Pat. No. 4,258,264. For the recording of the radiation image of a human body, such a stimulative photoradiation image recording and reproducing system is typically used as a system wherein a storage phosphor is employed in the form of a storage phosphor plate, the plate is exposed to an electromagnetic wave such as X-rays or a corpuscular ray, the image signal obtained through photo-electric reading is processed for obtaining the radiation image of a human body, and the radiation image as a visible image is reproduced on a material such as a photosensitive material, CRT or an image displaying sheet by use of the processed image signal. As the storage phosphor, there is typically used a phosphor which is obtained by dispersing a crystalline storage phosphor such as an alkali metal halide phosphor in a resin, a glass or a fiber in a porous state, as disclosed in Japanese Patent Provisional Publication No. 55(1980)-163472, Japanese Patent Provisional Publication No. 56(1981)-11392 and Japanese Patent Provisional Publication No. 56(1981)-11393. More specifically, in Japanese Patent Provisional Publication No. 55(1980)-163472, there is disclosed a light transmission type image displaying system wherein an X-ray image transmission film made of a storage phosphor is employed, the X-ray image transmission film is exposed to an X-ray pattern, and a light emitted in the portion exposed to the X-ray pattern is detected. In Japanese Patent Provisional Publication No. 56(1981)-11392, there is disclosed a method of forming an image wherein the storage phosphor plate is scanned with the light, and the light emitted by the phosphor is detected by a light detecting means. Also, in Japanese Patent Provisional Publication No. 56(1981)-11393, there is disclosed an X-ray image storage apparatus comprising a fluorescent plate in which a phosphor layer is formed on a film, and a film is formed on the surface of the phosphor layer, wherein the storage phosphor plate is scanned with the light and the fluorescent light emitted by the phosphor layer is detected. The conventional storage phosphor plate has an insufficient strength of the storage phosphor layer formed on a film such as a resin film, which leads to the difficulty of handling and transport. In order to solve the problem, a storage phosphor plate having a high mechanical strength has been proposed. For example, Japanese Patent Provisional Publication No. 1(1989)-133084 discloses a storage phosphor plate obtained by forming a phosphor layer on a support member in a thickness ranging from 0.1 to 5 mm, which is obtained by forming a phosphor layer on a support, said support being obtained by coating a water-insoluble resin having a glass transition point of from 10xc2x0 to 50xc2x0 C. with a fibrous material, followed by drying. However, such a storage phosphor plate is accompanied with a problem that the image quality obtained by scanning the plate with the stimulating light is deteriorated. The problem will hereinafter be described in detail. FIG. 7 is a schematic perspective view showing the structure of a conventional storage phosphor plate as disclosed in Japanese Patent Provisional Publication No. 1(1989)-133084. The storage phosphor plate has a phosphor layer 2 on a support member 1, the phosphor layer 2 being formed in a thin film by dispersing a phosphor in a binder, said binder being a polymer material having a glass transition point of from 10xc2x0 to 50xc2x0 C. The phosphor layer 2 has a thickness ranging from 0.1 to 5 mm, and is formed by spin coating or extrusion coating. In the conventional storage phosphor plate, a large stress is applied to the phosphor layer 2 during the steps of the production and the use. Particularly in case of using a phosphor having a low emission luminance such as a fluorescent lamp phosphor, a phosphor such as a rare earth activated alkaline earth metal fluoride phosphor or an X-ray storage phosphor, or a phosphor having a low mechanical strength such as the above described alkali metal halide phosphor as a phosphor, there is a problem that the emission luminance and the sensitivity of the phosphor are deteriorated by the stress. Also, as for the support member of the storage phosphor plate, glass is preferable in view of the transparency and the heat resistance. Glass, however, involves a problem that the glass is apt to be broken by the above described stress. The phosphor layer in the conventional storage phosphor plate has a two-layer structure, the phosphor layer being formed by the phosphor and a binder resin. Thus, the phosphor layer has an increased total thickness. Also, the phosphor layer is formed by dispersing the phosphor in the binder resin to form a phosphor layer solution, which is then coated on the support member, followed by drying, as disclosed in Japanese Patent Provisional Publication No. 1(1989)-133084. The phosphor is liable to be deposited in the coating solution during the coating and drying process to cause the problem of deteriorating the storage property of the phosphor. In addition, the phosphor layer formed by drying the phosphor layer solution after coating is limited in thickness. Further, the thickness of the phosphor layer is liable to vary. When the thickness of the phosphor layer is smaller, the phosphor layer becomes liable to be peeled off the support member, while when the phosphor layer is larger, the phosphor layer may be cracked or ruptured, causing the deterioration of the image quality. Further, when the phosphor layer is formed on the support member by coating the support member with a solution for forming the phosphor layer, the phosphor layer has a small strength, so that a problem may arise in that the phosphor layer is liable to be cracked when a shock is applied to the storage phosphor plate. Thus, in order to prevent the problem of the thickness of the phosphor layer, there is a proposed method of forming the phosphor layer in a thickness as thin as possible by allowing the phosphor to coagulate and deposit from a liquid phase, as disclosed in Japanese Patent Provisional Publication No. 4(1992)-191087. However, in the method disclosed in Japanese Patent Provisional Publication No. 4(1992)-191087, since a large quantity of the binder resin is required, the phosphor layer has a large thickness. In addition, a phosphor layer having a small thickness and a phosphor layer having a small stress are liable to be cracked during handling of the storage phosphor plate. Thus, the storage phosphor plate has various problems. Further, since the phosphor layer of the storage phosphor plate has a small strength, the storage phosphor plate is liable to suffer from the problem that the phosphor layer is cracked by a shock when the phosphor layer is handled. A typical phosphor layer of the storage phosphor plate has a two-layer structure obtained by coating a support member such as a plastic film with a solution for forming the phosphor layer, and is formed by dispersing the phosphor in the binder resin of a coating solution. The coating solution used for forming the phosphor layer is prepared by adding the phosphor in the form of a powder in a dispersing medium such as water to a binder resin such as polyvinyl alcohol, and the phosphor is dispersed in the binder resin in the coating solution. In order to form the phosphor layer, the coating solution is applied to the support member, followed by drying to form the phosphor layer. In case the phosphor is in the form of a powder, however, the phosphor tends to be deposited on the support member. Also, the phosphor is liable to be deposited on a surface of the support member which comes into contact with the phosphor to cause the problem that the thickness of the phosphor layer becomes non-uniform. When the thickness of the phosphor layer is larger, the phosphor layer tends to be cracked or ruptured due to the shock. Also, since the phosphor layer has a small strength, the phosphor layer is liable to suffer from the problem that the phosphor layer is cracked by a shock when the phosphor layer is handled. Further, when the phosphor layer is formed by using a dispersing medium such as water to which the phosphor is added, the dispersion medium is evaporated to volatilize the dispersion medium during the preparation of the coating solution, which leads to the problem that the phosphor layer has a small mechanical strength. In the conventional storage phosphor plate, the phosphor layer is formed in a thickness as small as several microns on the support member such as a plastic film. However, since the phosphor layer is formed by coating a support member with a phosphor layer solution and then drying the solution, the phosphor layer formed on the support member cannot have a thickness of less than several microns. In order to solve the above described problems of the conventional storage phosphor plate, it is an object of the present invention to provide a storage phosphor plate having a high mechanical strength