1. A radio tomographic image generation device comprising: a radiographic image obtaining unit, wherein at least one of a radiation source and a radiographic image detector for detecting radiation emitted from the radiation source and transmitted through a subject to output a radiographic image signal representing a radiographic image of the subject is made to orbit around the subject to obtain the radiographic image signal for each predetermined imaging angle outputted from the radiographic image detector when the radiation is applied to the subject at the predetermined imaging angle; and a tomographic image generation unit for generating a tomographic image of the subject by performing reconstruction using a successive approximation process based on the radiographic image signals for the individual imaging angles obtained by the radiographic image obtaining unit, wherein the tomographic image generation unit comprises: a reconstruction unit for generating a plurality of reconstructed images of different iteration numbers of the successive approximation process; a region segmentation unit for obtaining information about structure based on the radiographic image signals and segmenting, based on the information about structure, a region, of which the tomographic image is generated, into a plurality of segmented regions having different information about structure; and an image combining unit for generating partial tomographic images by using the reconstructed images of different iteration numbers for the individual segmented regions segmented by the segmentation unit based on the information about structure of the individual segmented regions, and generating the tomographic image of the subject by using the generated partial tomographic images for the individual segmented regions.