A particle beam irradiation system comprises deflection electromagnets 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 through which the particle beam passes so as to expand an energy width of the particle beam and form 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 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.