An X-ray computer tomography apparatus includes an X-ray tube 101, a two-dimensional array type X-ray detector 103, a rotating mechanism 102, a pair of collimators 131, 133, a collimator moving mechanism 135 which separately moves the pair of collimators in a direction almost parallel to a rotation axis, a reconstruction processing unit 114 which reconstructs image data in a reconstruction range,and a collimator control unit. The collimator control unit 121 controls the position of each collimator in accordance with the distance between an X-ray central plane corresponding to a cone angle 0 DEG 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 DEG to the opposite side.