According to one embodiment, an X-ray computed tomography apparatus includes an X-ray detector, data acquisition circuitry, and processing circuitry. The X-ray detector detects X-rays emitted by an X-ray tube and transmitted through an object and outputs an electrical signal concerning the detected X-rays. The data acquisition circuitry amplifies the electrical signal by a variable gain and acquires detection data based on the amplified electrical signal. The processing circuitry decides the gain and a modulation condition of a tube current in directional modulation scan based on body thickness information of the object.