A breast thickness measuring apparatus and a breast thickness measuring method are applied to a radiographic image capturing system. After a breast, which is placed on a support table, has been compressed by a compression plate having a marker disposed thereon, the breast is irradiated with radiation emitted from a radiation source. A radiographic image is generated on the basis of radiation that has passed through the breast, and a marker image, which is included within the radiographic image, is detected by a marker detector. Finally, the thickness of the compressed breast is calculated by a thickness calculator on the basis of a position of the detected marker image, a position of the radiation source, and information concerning the compression plate.