An X-ray CT apparatus according to an exemplary embodiment includes: a specifying unit that specifies the position of a lesion and the position of a surrounding site positioned in a surrounding of the lesion, from pieces of image data of the inside of the patient corresponding to the mutually-different temporal phases; a movement information calculating unit that calculates movement information related to movements of the lesion and the surrounding site, based on the positions of the lesion and the surrounding site specified by the specifying unit; and a relative relationship calculating unit that calculates a relative relationship between the movement information of the lesion and the movement information of the surrounding site calculated by the movement information calculating unit.