PURPOSE: To facilitate mounting and to assure an exact irradiation position by providing a freely attachable/detachable slice light holder in a beam source holding means and providing a light projecting part coinciding with a slice position in this slice light holder.

CONSTITUTION: A rotational mechanism part 15 is provided with a beam source holder 16. The ray source holding means 19 consisting of two pieces of parallel rails 18 which intersect orthogonally with the rotational mechanism part 15 and have hollow grooves 17 on the inner side is fixed to the beam source holder 16. The slice light holder 23 is also provided with projecting line parts 24, 24 on both sides like the beam source holder 16 and is provided with a handle 25 for attaching and detaching on the front. The light projecting part 26 is provided in the slice light holder 23. The light projecting part 26 is constituted of a light source 27 and an image formation lens 28 in a pair so as to coincide with the slice line when the light projecting part 26 is inserted to two pieces of the rails 18. Since the slice light holder 23 is installed in place of a rotary beam source for calibration only at the time of setting the slice position, the exact alignment thereof to the slide line of the detector is possible.