An X-ray CT apparatus according to an embodiment includes: an X-ray irradiator configured to emit X-rays to the examinee on a table; an X-ray detector configured to detect X-rays transmitted through the examinee on the table; a data collector configured to collect transmission data on those X-rays; a movement drive unit configured to move one of the table and the X-ray irradiator relative to the other in an outward direction and then in a homeward direction, the outward direction and homeward direction being opposite directions along a body axis of the examinee on the table; a position detector configured to detect a relative position between the table and the X-ray irradiator; and a data collection controller configured to control timings for starting and stopping data collection by the data collector, based on the relative position between the table and the X-ray irradiator.