An X-ray computed tomography apparatus includes an X-ray source (101) which generates X-rays, a high voltage generating unit (104) which generates a high voltage to be applied to the X-ray source, an X-ray detector (102) which detects X-rays transmitted through a subject to be examined to generate projection data, a storage unit (203) which stores the projection data in association with electrocardiographic data of the subject, a setting unit (207) which sets a specific cardiac phase in accordance with an operators instruction, a reconstruction unit (206) which reconstructs a tomogram on the basis of a plurality of projection data sets acquired in a plurality of specific periods centered on the specific cardiac phase throughout a plurality of cardiac cycles, a period extending unit (207) which extends the specific period on the basis of a heart rate fluctuation range of the subject, and a control unit (212) which controls 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.
