1. A digital radiography system comprising: an X-ray source irradiating an object to be inspected with X-ray; an X-ray image intensifier tube which receives the X-rays which pass through the object and converts the received X-rays into an optical image, a diameter of an input image of said X-ray image intensifier tube ranging from 305 to 406 mm, a diameter of an output image of said X-ray image intensifier tube ranging from 58 to 62 mm, and a ratio of the diameter of the input image to the diameter of the output image ranging from 5 to 7; a video camera which picks up the output image, said video camera having a fluoroscopic mode and a radiographic imaging mode, said fluoroscopic mode monitoring a real time X-ray image of the object irradiated by the X-rays, and said radiographic imaging mode recording an X-ray image of the object irradiated by X-rays, said video camera having an image pickup surface thereof which is the same for both said fluoroscopic mode and said radiographic imaging mode; an optical system including a plurality of lenses, said optical system being disposed between said X-ray image intensifier tube and said video camera so as to output substantially the same size output optical image of the X-ray image intensifier tube on the video camera in both of said fluoroscopic mode and said radiographic imaging mode; an image processor which converts an output from said video camera into digital image data; and an image display which displays an X-ray image by reading out said digital image data from said image processor.