A charged particle beam irradiation system includes measurement means 15 for measuring an accumulated beam charge amount Qm0, the ion beams orbiting in a synchrotron 13, immediately before an extraction control period in an operating cycle of the synchrotron 13 and beam extraction control means 20, 24, 28, 29 for controlling extraction of the ion beams so that extraction of a total of the ion beams is to be completed in time with expiration of an extraction control time Tex which is set in advance based on the measurement Qm0 of the accumulated beam charge amount. when an irradiation apparatus includes an RMW 32, an amplitude of an extraction radiofrequency voltage is controlled according to a ratio Qm0/Qs0 of the measurement to a reference value of the accumulated beam charge amount and a ratio Tb/Ta of an actual beam extraction time to the extraction control time Tex. This invention allows the beams accumulated in the synchrotron to be efficiently extracted and used and uniformity of the irradiation dose to be ensured.
