An objective of this invention is to give a dose distribution of higher precision to an irradiation target at a higher speed in a particle beam irradiation system. The particle beam irradiation system according to this invention includes deflection electromagnets for scanning which deflect the direction of a particle beam to make it scan and an energy width expanding device for expanding the energy width of the particle beam by letting the particle beam pass therethrough so as to form an SOBP in the irradiation direction of the particle beam to the irradiation target, that is the depth direction of the irradiation target. The energy width expanding device is so constituted that the SOBP in the depth direction is formed over the whole irradiation area in the depth direction of the irradiation taregt, and the deflection electromagnets are so controlled to make the irradiation spot of the particle beam formed in the irradiation target move stepwise over the whole irradiation area in the lateral direction of the irradiation target.
