PURPOSE: To enable control of irradiation area of an irradiation field even during electron irradiation by enabling moving of insulation blocks consisting of a pair of blocks in the directions perpendicular to each other.

CONSTITUTION: A particle beam 25 going out of a target 1 is directed by a primary collimator 3 and the particle beam 25 is output properly by measuring the irradiation rate of the particle beam 25 with a dose monitor 15. The particle beam 25 after passing through a film mirror is insulated for its unnecessary portion at the insulation surfaces 16c, 16d of an upper block 16. Then it is insulated similarly at the insulation surfaces 17c, 17d of a lower block 17. By moving the upper and the lower blocks 16, 17 to the directions of right and left or back and forth with a motor, the irradiation area of the particle beam 25 can be remotely controlled. The particle beam 25 thus irradiates in the irradiation field 11 in the shape of rectangular limited by the upper and the lower blocks 16, 17.