The charged particle beam irradiation system (1) includes a charged particle beam generating unit for accelerating a charged particle beam, scanning electromagnets for scanning the charged particle beam, a beam irradiation apparatus (5) for irradiating the accelerated charged particle beam to irradiation spots of an irradiation target, beam radiation dose measuring instrument (s) (59) for obtaining a dose of the charged particle beam passing through the beam irradiation apparatus, and a beam position measuring instrument for obtaining one or both of the position and the width of the beam scanned by the scanning electromagnets. The beam position measuring instrument (58) obtains one or both of the position and the width of the beam for each irradiation spot and determines whether the obtained result is within an allowable range and obtains one or both of the position and the width of the charged particle beam for each split during irradiation to the spot with the charged particle beam regarding a split of which a dose is managed by dividing a part of or all of irradiation spots into irradiation sections and determines whether the obtained result is within an allowable range.