According to one embodiment, an X-ray computed tomography apparatus includes an X-ray detection element, an A/D convertor, a readout switch, and readout control circuitry. The X-ray detection element outputs an electric signal corresponding to a detected X-ray. The A/D convertor A/D-converts the electric signal. The readout switch switches a connection between the X-ray detection element and the A/D convertor. The readout control circuitry acquires, as offset data, data which is output by the A/D convertor in a state where the readout switch is OFF instead of projection data which is output by the A/D convertor according to the electric signal in a state where the readout switch is ON, in a view during a scan.