Abstract: An X-ray control device is held in a constant position relative to an X-ray tube and an X-ray detector, while the X-ray tube is moved toward or away from the rotational scanning axis at which an object to be examined is located. The X-ray control device allows selection of a spot size and a tube voltage of the X-ray tube. The X-ray control device is controlled in synchronism with a shift mechanism. When the X-ray tube is close to the object, a smaller spot and a higher tube voltage than those when the X-ray tube is separated from the object are selected.