An X-ray computed tomography apparatus includes an X-ray source, a high voltage generating unit, an X-ray detector detecting X-rays transmitted through a subject to generate projection data, a storage unit storing the projection data in association with electrocardiographic data of the subject, a setting unit setting a specific cardiac phase in accordance with an operator's instruction, a reconstruction unit reconstructing a tomogram based on projection data sets acquired in specific periods centered on the specific cardiac phase throughout cardiac cycles, a period extending unit extending the specific period on the basis of a heart rate fluctuation range of the subject, and a control unit controlling the high voltage generating unit to generate a relatively high dose of X-rays in the extended specific period and generate a relatively low dose of X-rays in a period other than the extended specific period.