1. A breast thickness measuring device comprising: a support table on which a breast of a subject is placed; a compression plate that is displaced toward the support table in order to compress the breast; a first marker provided on the compression plate on a side of a chest wall of the subject; a second marker provided on the support table on the side of the chest wall of the subject; a radiation source that applies radiation from a plurality of different angles to the breast, which has been compressed; a radiation detector configured to generate a plurality of image data based on radiation that has been transmitted through the breast; a reconstruction processor configured to reconstruct the image data in order to generate a plurality of tomographic images; a marker detector configured to detect tomographic images that have captured the first marker and tomographic images that have captured the second marker, from among the tomographic images; a marker selector configured to select a tomographic image that is focused on the first marker from among the tomographic images that have captured the first marker, and selecting a tomographic image that is focused on the second marker from among the tomographic images that have captured the second marker; and a thickness calculator configured to calculate a thickness of the breast, which has been compressed, based on the tomographic image that is focused on the first marker and the tomographic image that is focused on the second marker.