There is provided a particle beam irradiation system so as to provide the dose distribution having more accuracy. An irradiation control part (13) comprises an energy setting controller (14) that sets the energy of a charged particle beam, a beam scanning controller (16) that controls a beam scanner (41), and a beam diameter changer (15) to control a beam diameter changer (40, 60), wherein the irradiation control part (13) sets a beam diameter of the charged particle beam to be a first beam diameter by the beam diameter changer (15), the charged particle beam is scanned step-wise by the beam scanning controller (16) so as to irradiate the charged particle beam on a predetermined region or the irradiation target, after that, the beam diameter of the charged particle beam is set by the beam diameter controller (15) to be a second beam diameter that is different from the first beam diameter, and the charged particle beam is scanned step-wise by the beam scan controller (16) so as to control the charged particle beam to irradiate on a region that is overlapped with at least a part of the predetermined part of the irradiation target.
