A charged particle beam irradiation system comprises a high-speed steerer (beam dump device) 100 disposed in a course of a beam transport line 4 through which an ion beam is extracted from a charged-particle beam generator 1. The beam dump device 100 is provided with dose monitoring devices 105, 106 for measuring a dose of an ion beam applied to a beam dump 104 so that the intensity of the ion beam can be measured without transporting the ion beam to irradiation nozzles 15A through 15D. Thus, the system is capable of adjusting the intensity of an ion beam extracted from a synchrotron without operating each component of a beam transport line, and an irradiation nozzle.