PROBLEM TO BE SOLVED: To solve the problem of excess irradiation in IMRT by a particle beam therapy apparatus by raising the irradiation flexibility in the depth direction, without utilizing a bolus.SOLUTION: A particle beam irradiation apparatus 58 has a scanning irradiation system 34 and a columnar irradiation field generation apparatus 4 which generates a columnar irradiation field and is mounted in a rotating gantry for rotating the irradiation direction of a charged particle beam 1. The columnar irradiation field generation apparatus generates according to the distal shape of an irradiation object an outer columnar irradiation field which is a columnar irradiation field having one SOBP width, and an inner columnar irradiation field, inside the irradiation object, which is a columnar irradiation field having another SOBP width different from the one for the outer columnar irradiation field. The scanning irradiation system scans and irradiates the outer columnar irradiation field according to the distal shape of the irradiation object in a spread-all-over fashion, and scans and irradiates the inner columnar irradiation field as if spreading the inner irradiation field all over the inside of the outer irradiation field.
