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 that takes an image of an outer end of a respective downstream side face of a leaf plate is provided for each row of leaves in such a way as to be situated at a position that is at an outer side of an irradiation field and adjusted in such a way that the base of a perpendicular from a viewpoint to the downstream side face, is situated at a position that is at an inner side of the position of the outer end when the leaf plate is maximally driven in the departing direction.
