This invention aims to provide a dose distribution with higher accuracy at higher speed in a particle beam irradiation system.A particle beam irradiation system according to this invention comprises deflection electromagnets for scanning which scan by deflecting the particle beam in two dimensions in the lateral direction which is perpendicular to an irradiation direction of the particle beam, and an energy width expanding device which expands an energy width of the particle beam and forms a SOBP in a depth direction of the irradiation target, that is, in an irradiation direction of the particle beam, wherein the energy width expanding device is configured to form the SOBP in the depth direction along whole irradiation area in the depth direction of the irradiation target, and the deflection electromagnets for scanning are controlled so as for an irradiation spot which is formed in the irradiation target by the particle beam to move stepwise along whole irradiation area in the lateral direction of the irradiation target.
