1. A tomographic image obtainment apparatus, wherein a tomographic image is reconstructed from a plurality of radiographic images obtained by irradiating a subject with radiation from various directions, the apparatus comprising: a radiation image detector for obtaining radiographic images of the subject; a radiation irradiation unit placed so as to face the radiation image detector, the radiation irradiation unit moving to a plurality of positions and irradiating, at the plurality of positions, the subject placed on the radiation image detector with radiation from various directions; an overlap-degree obtainment means for obtaining the degree of overlap of anatomical structures of the subject; and an exposure condition setting means for setting, based on the degree of overlap obtained by the overlap-degree obtainment means, a condition of exposure by the radiation irradiation unit at the plurality of positions to obtain the plurality of radiographic images.