According to one embodiment, an X-ray computed tomography apparatus includes an X-ray tube, a high voltage power supply, and focus size control circuitry. The X-ray tube includes a cathode, an anode, and a deflector configured to deflect the electrons from the cathode. The high voltage power supply generates a tube voltage to be applied between the cathode and the anode. The focus size control circuitry controls a focus size formed in the anode by applying to the deflector a deflecting voltage of a deflecting voltage value based on a tube voltage value of the tube voltage and a predetermined size, in order to form a focus of the predetermined size in the anode during the period where the tube voltage is applied by the high voltage power supply.