There is provided a particle beam irradiation system so as to provide the dose distribution having more accuracy. An irradiation control part comprises an energy setting controller that sets the energy of a charged particle beam, a beam scanning controller that controls a beam scanner, and a beam diameter changer to control a beam diameter changer, wherein the irradiation control part sets a beam diameter of the charged particle beam to be a first beam diameter by the beam diameter changer, the charged particle beam is scanned step-wise by the beam scanning controller so as to irradiate the charged particle beam on a predetermined region of the irradiation target, after that, the beam diameter of the charged particle beam is set by the beam diameter controller 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 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.