A particle beam scanning irradiation system includes a computer establishing a scanning sequence for irradiation of a tumor portion in a patient and a particle beam irradiation device irradiating the tumor portion in accordance with the established scanning sequence of the particle beam. The computer selects all conceivable combinations of pairs of irradiation spots among the plurality of irradiation spots arranged in the tumor portion, and determines whether each path for the particle beam to shift between two spots constituting the selected pair passes through the tumor portion determines a penalty matrix expressing whether each path passes through the tumor portion on the basis of the determination result evaluates a function for the shift paths on the basis of an optimizing algorithm, and establishes the scanning sequence of the particle beam by an optimized solution of the function.
