PURPOSE: To eliminate the leakage of radiant rays passing through the opposed shielding surfaces of shielding bodies, when the opposed shielding bodies are closed, and to execute the radiation therapy with high accuracy by providing the projecting surface and the recessed surface so as to be engaged to each other in a step difference on the opposed shielding surfaces of the shielding bodies.

CONSTITUTION: The device is provided with shielding bodies 2a, 2b which are driven centering around a radiation source 1 and so as to move in the peripheral side face of a virtual cylindrical body having a center line containing said source, and on the shielding body 2a and the shielding body 2b, the projecting shielding surface 3a and the recessed surface 3a, and the projecting shielding surface 4a and the recessed surface 4c are formed so that the projecting surface and the recessed surface are opposed to each other. This device is provided with a diaphragm 7 having a conical opening part 7a for limiting the maximum radiation field of end faces 5a, 5b of the radiation source 1 side of the shielding bodies 2a, 2b, and radiation field side end faces 6a, 6b of the shielding bodies 2a, 2b, and also, obstructing the radiation of radiant rays to an unnecessary part. In such a way, as for cutting-off of radiant rays, the radiant rays are cut off effectively by engaging the projecting surface and the recessed surface engaged to each other in a step difference of the opposed surfaces of the opposed shielding bodies in the radiation transmission direction so that the radiant rays do not pass through.