1. A radiation tomography apparatus comprising: a radiation source for irradiating a subject with radiation; a collimator for collimating radiation from the radiation source; a radiation detecting device for detecting radiation applied to the subject; a top board between the radiation source and the radiation detecting device for supporting the subject placed thereon; a moving device for moving the radiation source and the radiation detecting device synchronously in opposite directions to each other along the top board; a movement controlling device for controlling the moving device; an image generating device for generating a fluoroscopic image in accordance with detection signals outputted from the radiation detecting device; a boundary identifying device for identifying from the fluoroscopic image a boundary between a shadow area in the fluoroscopic image and a non-shadow area, in the shadow area a shadow of the collimator appearing and in the non-shadow a shadow of the collimator not appearing; an image complement device for generating a complement image through complementing the shadow area in the fluoroscopic image with a complement value with reference to the boundary , the complement value indicating a pixel value of the non-shadow area or indicating a pixel value nearer to that of the non-shadow area than that of the shadow area; and a superimposing device for superimposing a series of the complement images generated from a series of the fluoroscopic images that are continuously taken while the radiation source and the radiation detecting device move to generate a tomographic image.