An X-ray computer tomography apparatus includes an X-ray tube, a two-dimensional array type X-ray detector, a rotating mechanism, a pair of collimators, a collimator moving mechanism which separately moves the pair of collimators in a direction almost parallel to a rotation axis, a reconstruction processing unit which reconstructs image data in a reconstruction range, and a collimator control unit. The collimator control unit controls the position of each collimator in accordance with the distance between an X-ray central plane corresponding to a cone angle 0° and an end face of the reconstruction range. The collimator moving mechanism moves each of the pair of collimators in the range from the outermost position corresponding to the maximum cone angle to the innermost position offset from a position corresponding to a cone angle of nearly 0° to the opposite side.