Abstract: A particle beam irradiation system which can increase an availability factor. An ion beam extracted from one proton beam linac is bent at 90 degrees by a switching magnet and is introduced to RI production equipment through a beam line. In the RI production equipment, a RI is produced using the introduced ion beam. An ion beam extracted from the other proton beam linac is bent at 90 degrees by the switching magnet and is introduced to a synchrotron through a beam line. The ion beam extracted from the synchrotron is irradiated to a patient from irradiation equipment. If one proton beam linac comes into an abnormal state, the one proton beam linac is stopped in operation and checked. During the check, the ion beam extracted from the other proton beam linac is selectively introduced to the RI production equipment and the synchrotron by the switching magnet.