1. A charged particle beam irradiation system, comprising: a synchrotron which accelerates an ion beam, wherein the ion beam is extracted from the synchrotron; an irradiation apparatus for irradiating an object to be irradiated with the ion beam introduced from the synchrotron; detection means for measuring an amount of accumulated charge of the ion beam that orbits in the synchrotron immediately before an extraction control period in an operating cycle of the synchrotron; and beam extraction control means for controlling extraction of the ion beam based on the measurement result of the accumulated beam charge amount so that extraction of a total amount of the ion beam is to be completed with an expiration of an extraction control time, the extraction control time representing a length of the extraction control period of the synchrotron and being set in advance, wherein: the irradiation apparatus is structured to operate at a predetermined irradiation cycle, and the beam extraction control means sets a value of the integral multiple of the irradiation cycle of the irradiation apparatus for the extraction control time such that an end of the extraction control time occurs at an end of a last irradiation cycle of the irradiation apparatus that occurs within the extraction control time.