1. A radiation tomographic image generating method for generating tomographic images based on a plurality of radiological images acquired by radiation beams emitted from different directions to an inspection object, respectively, the radiation tomographic image generating method comprising: an extraction separation step for extracting and separating, from the acquired radiological images, images of a substance extraneous to a substance forming an area of interest in the inspection object; an area interpolation step for generating interpolation images by interpolating an area of the extraneous substance, from peripheral regions thereof, in the images of the extraneous substance extracted and separated in the extraction separation step; a first tomographic image generating step for generating tomographic images from the interpolation images generated with the area of the extraneous substance interpolated in the area interpolation step; an extraneous projection data generating step for generating extraneous projection data which is projection data of the extraneous substance, from differences between the acquired radiological images and the interpolation images; a second tomographic image generating step for generating tomographic images from the extraneous projection data generated in the extraneous projection data generating step; and a tomographic image synthesizing step for synthesizing the tomographic images generated in the first tomographic image generating step and the tomographic images generated in the second tomographic image generating step.