A charged particle beam generator, a charged particle irradiation system, a method for operating the charged particle beam generator and a method for operating the charged particle irradiation system, which allow a charged particle beam to be injected into a circular accelerator at an arbitrary timing and can reduce an irradiation time and a time for a therapy, are provided while maintaining the lower limit of an operation cycle of a linear accelerator. An accelerator control device controls an operation of a synchrotron on the basis of a beam extraction request signal transmitted from a beam utilization system control device. A control device generates a timing signal notifying the linear accelerator of an injection timing of a next operation cycle of the synchrotron after completion of an extraction process performed by the synchrotron, changes an operation timing of the linear accelerator so that the operation timing of the linear accelerator matches the injection timing.