A mammographic CT apparatus includes: a gantry, including a front face plate in which an insertion opening is formed for inserting a breast into an accommodation portion a radiation tube and sensing device disposed within the gantry; a driving unit configured to rotate the radiation tube and the sensing device around a rotation axis set in the normal direction of the accommodation portion, at the same angular speed and in the same direction; a collimator disposed between the accommodation portion and the radiation tube, and configured to rotate around the rotation axis integrally with the radiation tube and sensing device; and an annular shield disposed between the rotational path of the collimator and the rotational path of the radiation tube, having an annular gap corresponding to the rotational path of the collimator, wherein an edge of the collimator or sensing device is fit into the gap.