A mammography apparatus capable of obtaining a breast image having an optimum density with reduced burden on the subject. The apparatus includes a radiation irradiation section and an object table connected to an arm such that they face each other, and the breast compressed onto the object table using a compression paddle is imaged by rotating the arm by an arm rotation means according to the imaging direction. In the apparatus, the size of the breast is detected, and the position of a dose detector is moved along the side of the object table facing the chestwall of the subject according to the detected size of the breast and the angle of rotation of the arm when rotated by the arm rotation means.