A particle beam scanning irradiation system (100) includes a computer (50) establishing a scanning sequence for irradiation of a tumor portion in a patient with a particle beam and a particle beam irradiation device (20) irradiating the tumor portion in the patient with the particle beam in accordance with the established scanning sequence of the particle beam. The computer (50) executes a step of selecting all conceivable combinations of pairs of irradiation spots among the plurality of irradiation spots arranged in the tumor portion, and determining whether or not each of shift paths for the particle beam to shift between two irradiation spots constituting the selected pair passes through the tumor portion a step of determining a penalty matrix having matrix elements expressing whether or not each shift path passes through the tumor portion on the basis of the determination result and a step of evaluating an evaluation function for the shift paths on the basis of an optimizing algorithm, and establishing the scanning sequence of the particle beam by an optimized solution of the evaluation function obtained from the evaluation.
