A particle beam therapy system comprising: a synchrotron for accelerating a charged particle beam including an acceleration cavity adapted to apply a radio frequency acceleration voltage such that the charged particle beam has a predetermined energy level, an extraction device adapted to cause the charged particle beam to exceed a stability limit, and to extract the thus-prepared charged particle beam; a beam transport system for introducing the charged particle beam extracted from said synchrotron to a treatment room; an irradiation system for irradiating an affected area of a patient with the charged particle beam in conformity with the shape of the affected area in the treatment room; and a controller adapted to turn on the extraction device when the charged particle beam is to be supplied to said irradiation system; characterised in that the extraction device is adapted to apply a radio frequency electromagnetic field to enlarge the emittance of the charged particle beam when extracting the charged particle beam from the synchrotron; the controller is adapted to turn off the radio frequency electromagnetic field of the extraction device when the supply of the charged particle beam to said irradiation system is to be blocked by means of an electromagnet provided in said beam transport system or in said synchrotron, and to turn off the acceleration cavity in synchronization with the turning off of the radio frequency electromagnetic field of the extraction device when the supply of the charged particle beam to the irradiation system is to be blocked. a synchrotron for accelerating a charged particle beam including an acceleration cavity adapted to apply a radio frequency acceleration voltage such that the charged particle beam has a predetermined energy level, an extraction device adapted to cause the charged particle beam to exceed a stability limit, and to extract the thus-prepared charged particle beam; an acceleration cavity adapted to apply a radio frequency acceleration voltage such that the charged particle beam has a predetermined energy level, an extraction device adapted to cause the charged particle beam to exceed a stability limit, and to extract the thus-prepared charged particle beam; a beam transport system for introducing the charged particle beam extracted from said synchrotron to a treatment room; an irradiation system for irradiating an affected area of a patient with the charged particle beam in conformity with the shape of the affected area in the treatment room; and a controller adapted to turn on the extraction device when the charged particle beam is to be supplied to said irradiation system; characterised in that the extraction device is adapted to apply a radio frequency electromagnetic field to enlarge the emittance of the charged particle beam when extracting the charged particle beam from the synchrotron; the controller is adapted to turn off the radio frequency electromagnetic field of the extraction device when the supply of the charged particle beam to said irradiation system is to be blocked by means of an electromagnet provided in said beam transport system or in said synchrotron, and to turn off the acceleration cavity in synchronization with the turning off of the radio frequency electromagnetic field of the extraction device when the supply of the charged particle beam to the irradiation system is to be blocked.