A subject information acquisition unit of a mammary gland amount calculation unit acquires the thickness and the mammary gland amount of the breast based on a low voltage image. An absorption coefficient acquisition unit acquires an absorption coefficient of fat and an absorption coefficient of mammary gland with reference to the relationship between the thickness of the breast and the X-ray absorption coefficient, which is calculated in advance according to the energy distribution and the substance forming the breast. A calculation unit calculates a new thickness and a new mammary gland amount of the breast based on the low voltage image, a high voltage image, a tube voltage at the time of imaging, the absorption coefficient of fat, and the absorption coefficient of mammary gland.
