PURPOSE: To visually observe the slice surface of a body to be examined so that its operation can be monitored by providing an annular X-ray shielding body for passing through the inside peripheral part side of an X-ray detector array, and executing a reciprocating motion along its center axis, and arranging a reflecting mirror in the inside peripheral part of the shielding body.

CONSTITUTION: X rays are radiated from an X-ray tube 10 on the X-ray track periphery (a), transmit through a body to be examined and are made incident on a detector 12 on the detector periphery (b). Light from a projector 18 is reflected by a reflecting mirror 14M attached to a shutter 14, and projects a light spot to the body to be examined. By this light spot, a relative position of a patient and the device is varied by moving a gantry and a bed, etc., and positioning is executed. Immediately after turning on a filament of the X-ray tube, X-ray radiation from the X-ray tube is unstable, therefore, usually the X-ray tube is subjected to electric conduction in a position (position of A) in which a shutter 14 is closed, and after the X-ray radiation becomes stable, the shutter 14 is opened (position of B) and data is collected. After the data is collected, the shutter 14 is set to the position of A again, and closed, by which the X ray is shielded.