A particle beam therapy system comprises a charged particle beam generator for generating a charged particle beam, two or more treatment rooms provided with respective irradiation devices for irradiating the charged particle beam, a beam line for transporting the charged particle beam extracted from the charged particle beam generator to the irradiation device in selected one of the two or more treatment rooms, a beam detection processing/control unit for monitoring a beam state of the charged particle beam in one of the two or more irradiation devices, and a selector for switchably selecting one of the irradiation devices which is to be monitored by the beam detection processing/control unit. The selector is controlled such that the selector establishes connection with the irradiation device in the selected one treatment room to which the charged particle beam is transported through the beam line. The system configuration can be simplified while maintaining the operation efficiency.