The objective is to obtain a particle beam therapy system that can suppress the effect of a leakage dose. There are provided a scanning nozzle (21) that irradiates in a predetermined direction a particle beam (B) emitted from an accelerator (1) an irradiation control unit (42) that controls operation of the irradiation nozzle (21) in such a way that each of a plurality of spots (SP) set in a planar direction (ASS) in an irradiation subject (TC) is sequentially irradiated with a particle beam (B) of a predetermined dose and a control unit (4) that on/off-controls emission of the particle beam (B) from the accelerator (1). The irradiation control unit (42) makes the irradiation nozzle (21) scan in a diluting manner the particle beam (B) onto a predetermined area (AP) in the irradiation subject (TC), in a predetermined period (PW) after a time point (TF) when emission is switched from ON to OFF, or in a period from the time point when emission is switched from ON to OFF to a time point when the particle beam (B) is cut off.
