Patent Number: 
Section: claims

1. A particle beam therapy system comprising:an irradiation nozzle that scans and irradiates a particle beam supplied from an accelerator, by use of two electromagnets whose scanning directions are different from each other, said two electromagnets including an upstream electromagnet and a downstream electromagnet; anda multileaf collimator that is disposed on a beam orbit of a particle beam irradiated from the irradiation nozzle and that limits or forms an irradiation field of the charged particle beam in such a way that the irradiation field conforms to the shape of an irradiation subject, the multileaf collimator comprising:a leaf row in which a plurality of leaf plates are arranged in a thickness direction thereof in such a way that respective inner end faces of the leaf plates are trued up; anda leaf plate drive mechanism configured to (i) drive each of the plurality of leaf plates in such a way that an inner end face approaches or departs from a beam axis of the charged particle beam and (ii) drive the plurality of leaf plates along a circumferential orbit around a scanning axis of the downstream electromagnet and at a preset distance from the scanning axis of the downstream electromagnet, wherein the particle beam is irradiated through a scanning method. 2. The particle beam therapy system according to claim 1,wherein in each of the plurality of leaf plates, a facing side facing a leaf plate that is adjacent to that leaf plate in the thickness direction is formed of a plane including a scanning axis of the upstream electromagnet, which is perpendicular to the beam axis and is set at a first position on the beam axis, andwherein the circumferential orbit along the scanning axis of the downstream electromagnet, around which the plurality of leaf plates are driven, is perpendicular to the beam axis and the scanning axis of the upstream electromagnet and is set at a second position that is on the beam axis and separates from the first position by a predetermined distance. 3. The particle beam therapy system according to claim 2, wherein the respective inner end faces of the plurality of leaf plates are on a plane including the scanning axis of the downstream electromagnet. 4. The particle beam therapy system according to claim 2, wherein the respective inner end faces of the plurality of leaf plates are driven by the leaf plate drive mechanism in such a way as to be on a plane including the scanning axis of the downstream electromagnet. 5. The particle beam therapy system according to claim 1, wherein each of the leaf plates of the plurality of leaf plates has four end faces, and wherein an incident-side end face and an emitting-side end face, among the four end faces, which are adjacent to the inner end face, are formed in the shape of an arc whose center is the scanning axis of the downstream electromagnet.