A particle beam irradiation system comprising: - a particle beam generation part; - a particle beam irradiation part where a charged particle beam that is generated in the particle beam generation part is irradiated on an irradiation target, the irradiation target comprising a plurality of irradiation layers (Zi, Zi+1) wherein each irradiation layer (Zi, Zi+1) is irradiated with a different charged particle beam energy (Ei, Ei+1) and; - an irradiation control part that is adapted to control the charged particle beam to be irradiated; - wherein the particle beam irradiation part comprises: - a beam scanner that is adapted to scan the charged particle beam laterally in two dimensions that are perpendicular to the irradiation direction of the charged particle beam; and - a beam diameter changer (40, 60) that is adapted to change the beam diameter of the charged particle beam; - wherein the irradiation control part comprises: - an energy setting controller that is adapted to set the energy of the charged particle beam; - a beam scanning controller that is adapted to control the beam scanner; and - a beam diameter controller that is adapted to control the beam diameter changer (40, 60); - characterized in that: - when the irradiation control part sets the beam diameter of the charged particle beam by the beam diameter controller to be a first beam diameter, the charged particle beam is adapted to be scanned step-wise by the beam scanning controller so as to irradiate the charged particle beam on the whole of a predetermined irradiation layer (Zi) of the irradiation target to be scanned laterally in two dimensions, and when the beam diameter of the charged particle beam is set by the beam diameter controller to be a second diameter that is smaller than the first beam diameter, the charged particle beam is adapted to be scanned step-wise by the beam scanning controller so as to control the charged particle beam to irradiate on a region that overlaps only with a part of the whole predetermined irradiation layer (Zi) of the irradiation target to be scanned laterally in two dimensions. - a particle beam generation part; - a particle beam irradiation part where a charged particle beam that is generated in the particle beam generation part is irradiated on an irradiation target, the irradiation target comprising a plurality of irradiation layers (Zi, Zi+1) wherein each irradiation layer (Zi, Zi+1) is irradiated with a different charged particle beam energy (Ei, Ei+1) and; - an irradiation control part that is adapted to control the charged particle beam to be irradiated; - wherein the particle beam irradiation part comprises: - a beam scanner that is adapted to scan the charged particle beam laterally in two dimensions that are perpendicular to the irradiation direction of the charged particle beam; and - a beam diameter changer (40, 60) that is adapted to change the beam diameter of the charged particle beam; - wherein the irradiation control part comprises: - an energy setting controller that is adapted to set the energy of the charged particle beam; - a beam scanning controller that is adapted to control the beam scanner; and - a beam diameter controller that is adapted to control the beam diameter changer (40, 60); - characterized in that: - when the irradiation control part sets the beam diameter of the charged particle beam by the beam diameter controller to be a first beam diameter, the charged particle beam is adapted to be scanned step-wise by the beam scanning controller so as to irradiate the charged particle beam on the whole of a predetermined irradiation layer (Zi) of the irradiation target to be scanned laterally in two dimensions, and when the beam diameter of the charged particle beam is set by the beam diameter controller to be a second diameter that is smaller than the first beam diameter, the charged particle beam is adapted to be scanned step-wise by the beam scanning controller so as to control the charged particle beam to irradiate on a region that overlaps only with a part of the whole predetermined irradiation layer (Zi) of the irradiation target to be scanned laterally in two dimensions. - when the irradiation control part sets the beam diameter of the charged particle beam by the beam diameter controller to be a first beam diameter, the charged particle beam is adapted to be scanned step-wise by the beam scanning controller so as to irradiate the charged particle beam on the whole of a predetermined irradiation layer (Zi) of the irradiation target to be scanned laterally in two dimensions, and when the beam diameter of the charged particle beam is set by the beam diameter controller to be a second diameter that is smaller than the first beam diameter, the charged particle beam is adapted to be scanned step-wise by the beam scanning controller so as to control the charged particle beam to irradiate on a region that overlaps only with a part of the whole predetermined irradiation layer (Zi) of the irradiation target to be scanned laterally in two dimensions.