A particle beam therapy system, comprising: an irradiation nozzle configured to scan a particle beam supplied from an accelerator, with two electromagnets (1a, 1b) whose scanning directions are different from each other, the accelerator being configured to irradiate the particle beam; a multi-leaf collimator disposed in a path of the particle beam irradiated from the irradiation nozzle and adapted to limit or form an irradiation field of the particle beam so that said irradiation field conforms to the shape of an irradiation subject, the multi-leaf collimator comprising: a leaf row in which a plurality of leaf plates are arranged in the thickness direction thereof (x) in such a way that the respective end faces of the leaf plates are trued up; and a leaf plate drive mechanism configured to drive each of the plurality of leaf plates in such a way that the end face approaches or departs from a beam axis of the particle beam, the end face facing the irradiation field; characterized in that: in each of the leaf plates, a facing side facing a leaf plate that is adjacent to that leaf plate in the thickness direction (x) is formed to be comprised in a plane (Psa) including a first axis (Asa), said first axis (Asa) being perpendicular to the beam axis and set at a first position (CPa) on the beam axis, in that the leaf plate drive mechanism is configured to drive the leaf plate along a circumferential orbit around a second axis (Asb) that is perpendicular to the beam axis and the first axis (Asa) and is set at a second position (CPb) that is on the beam axis, the second position (CPb) being separated from the first position (CPa) with a predetermined distance, and in that the respective end faces of the leaf plates are comprised in a plane including the second axis (Asb). an irradiation nozzle configured to scan a particle beam supplied from an accelerator, with two electromagnets (1a, 1b) whose scanning directions are different from each other, the accelerator being configured to irradiate the particle beam; a multi-leaf collimator disposed in a path of the particle beam irradiated from the irradiation nozzle and adapted to limit or form an irradiation field of the particle beam so that said irradiation field conforms to the shape of an irradiation subject, the multi-leaf collimator comprising: a leaf row in which a plurality of leaf plates are arranged in the thickness direction thereof (x) in such a way that the respective end faces of the leaf plates are trued up; and a leaf plate drive mechanism configured to drive each of the plurality of leaf plates in such a way that the end face approaches or departs from a beam axis of the particle beam, the end face facing the irradiation field; a leaf row in which a plurality of leaf plates are arranged in the thickness direction thereof (x) in such a way that the respective end faces of the leaf plates are trued up; and a leaf plate drive mechanism configured to drive each of the plurality of leaf plates in such a way that the end face approaches or departs from a beam axis of the particle beam, the end face facing the irradiation field; in each of the leaf plates, a facing side facing a leaf plate that is adjacent to that leaf plate in the thickness direction (x) is formed to be comprised in a plane (Psa) including a first axis (Asa), said first axis (Asa) being perpendicular to the beam axis and set at a first position (CPa) on the beam axis, in that the leaf plate drive mechanism is configured to drive the leaf plate along a circumferential orbit around a second axis (Asb) that is perpendicular to the beam axis and the first axis (Asa) and is set at a second position (CPb) that is on the beam axis, the second position (CPb) being separated from the first position (CPa) with a predetermined distance, and in that the respective end faces of the leaf plates are comprised in a plane including the second axis (Asb).