An X-ray CT apparatus has a setting unit, a generating unit, a calculation unit, and a determination unit. The setting unit sets a required width of a group of X-ray detection elements in a row direction, which is relatively insusceptible to an influence of a cone angle of a cone-beam X-ray. The generating unit generates accumulation data by accumulating first projection data, based on first X-ray detection elements within the required width included in the group, with same channel in the row direction in preference to second projection data based on second X-ray detection elements outside the required width included in the group. The calculation unit obtains a motion amount for each heartbeat phase based on the accumulation data. The determination unit determines a specific heartbeat phase based on the motion amount for the each heartbeat phase.