The objective is to obtain a particle beam therapy system that prevents dispersion of a charged particle beam, reduction of the energy thereof, and upsizing of the system and that can accurately monitor the opening shape of a multileaf collimator so as to perform high-accuracy particle beam therapy. An image-capturing unit (11) that takes image of outer end of respective downstream side face of a leaf plate is provided for each of leaf rows (8C) in such a way as to be situated at a position that is at outer side of an irradiation field (RB) and adjusted in such a way that foot of a perpendicular from a viewpoints (PV) to the downstream side face, is situated at a position that is at inner side of the position of the outer end when the leaf plate is maximally driven in the departing direction.
