Multi-leaf collimator, particle beam therapy system, and treatment planning apparatus

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;
• and
• a 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;
• and
• a 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.