An X-ray imaging apparatus irradiates an X-ray every predetermined rotation angle of an arc arm for oppositely supporting an X-ray tube and an X-ray detector through a subject while the arc arm is reciprocated and rotated over a predetermined angle range. The cam portion is adapted to engage an item to be clamped on the base. In addition, the retirement control block further controls the retiring of a group of instructions determined to be retirable, by simultaneously transferring their results from the temporary buffer to the register array, and retires instructions executed in order by storing their results directly in the register array.