In a collimator 6 according to the present invention, an irradiation field F is accurately set with respect to an affected area T by making a plurality of leaf plates 13B be slidable along a left-right direction X. Further, by making separate leaf plates 15 of the leaf plates 13B be slidable in an irradiation direction Y as well, it is possible to bring the separate leaf plates 15 close to the body surface of a patient P so as to conform to the outer shape of an irradiation site. A gap between the collimator 6 and the patient P is narrowed thereby, whereby dispersion of neutrons is suppressed. As a result, a neutron dose which is irradiated to the affected area T per hour is secured, and thus irradiation time is shortened.
