When IMRT technology for a radiation therapy system utilizing an X-ray or the like is applied to a particle beam therapy system having a conventional wobbler system, it is required to utilize two or more boluses. The present invention solves the problem of excess irradiation in IMRT by a particle beam therapy system. More specifically, the problem of excess irradiation in IMRT by a particle beam therapy system is solved by raising the irradiation flexibility in the depth direction, without utilizing a bolus. A particle beam irradiation apparatus has a scanning irradiation system that performs scanning with a charged particle beam accelerated by an accelerator and is mounted in a rotating gantry for rotating the irradiation direction of the charged particle beam. The particle beam irradiation apparatus comprises a columnar-irradiation-field generation apparatus that generates a columnar irradiation field by enlarging the Bragg peak of the charged particle beam.
