1. Radiographic apparatus comprising: a radiation source for emitting radiation; a radiation source controlling device for controlling output of radiation; a radiation detecting device for detecting radiation; a moving device for moving the radiation source and the radiation detecting device synchronously; an image generation device for generating a fluoroscopic image based on a detection signal from the radiation detecting device; and a superimposing device for superimposing a series of fluoroscopic images continuously taken while the radiation source and the radiation detecting device moves or superimposes an identical site of the subject in the series of fluoroscopic images one another, thereby generating a tomographic image, the radiation detecting device comprising: (A) a radiation conversion device for converting radiation entering the radiation detecting device into an electric signal; and (B) an amplifying device for amplifying the electric signal by a given amplification factor, wherein letting an imaging mode where the radiation detecting device is irradiated with single shot radiation to obtain a single fluoroscopic imaging be a spot radiography mode, and an imaging mode where a tomographic image is obtained be a tomography mode, an amplification factor is higher in the tomography mode than in the spot radiography mode, and output of single-shot radiation from the radiation source is less in the tomography mode than in the spot radiography mode.