Abstract: An X-ray CT apparatus according to an embodiment includes: a table on which an examinee lies down; an X-ray irradiator including an X-ray tube emitting X-rays to the examinee on the table and a diaphragm blocking the X-rays and capable of opening/closing; an X-ray detector detecting X-rays transmitted through the examinee on the table after being emitted by the X-ray irradiator and passing through the diaphragm; a rotating gantry supporting the X-ray irradiator and detector; a rotation drive unit rotating the rotating gantry about a body axis of the examinee on the table; a movement drive unit moving the table in a direction of the body axis of the examinee on the table; a positional information acquirer acquiring positional information on the moving table; and a controller controlling opening and closing operations of the diaphragm of the X-ray irradiator based on the positional information on the table.