A charged particle beam irradiation system comprising: a charged particle beam generator adapted to accelerate and extract a charged particle beam, wherein said charged particle beam generator includes a synchrotron, and a radiofrequency-wave applicator located in the synchrotron adapted to apply a radiofrequency wave to a charged particle beam orbiting in the synchrotron; a first beam transport line adapted to transport the charged particle beam extracted from the charged particle beam generator, said first beam transport line being connected to the charged particle beam generator; a plurality of irradiation nozzles (15A-D; 15E-H) adapted to irradiate an affected part of a patient with the charged particle beam; a plurality of second beam transport lines (5A-D) branching from the first beam transport line and connected to the respective irradiation nozzles (15A-D; 15E-H); a plurality of switching magnets (6A-C) disposed on the first beam transport line and adapted to selectively introduce the charged particle beam extracted from the charged particle beam generator into one of the second beam transport lines (5A-D); a beam dump device including a steering magnet that is disposed on the first beam transport line and a beam dump that is disposed on a beam path branching from the steering magnet, said beam dump device being adapted to control the excitation of the steering magnet such that from among charged particle beams extracted from the charged particle beam generator, a charged particle beam which is not to be transported to one of the plurality of irradiation nozzles (15A-D; 15E-H) is applied to the beam dump; and a dose monitoring device disposed in the beam dump device and adapted to measure the dose of the charged particle beam applied to the beam dump, characterized in that said charged particle beam generator further includes a voltage regulator adapted to control a radiofrequency voltage to be supplied to the radiofrequency-wave applicator on the basis of a predetermined gain pattern of the radiofrequency voltage and for controlling the intensity of a charged particle beam extracted from the synchrotron to the first beam transport line, and the charged particle beam irradiation system further comprises a voltage gain pattern creation device (30, 200b) adapted to create a voltage gain pattern to be set for the voltage regulator based on a measured value by the dose monitoring device. a charged particle beam generator adapted to accelerate and extract a charged particle beam, wherein said charged particle beam generator includes a synchrotron, and a radiofrequency-wave applicator located in the synchrotron adapted to apply a radiofrequency wave to a charged particle beam orbiting in the synchrotron; a first beam transport line adapted to transport the charged particle beam extracted from the charged particle beam generator, said first beam transport line being connected to the charged particle beam generator; a plurality of irradiation nozzles (15A-D; 15E-H) adapted to irradiate an affected part of a patient with the charged particle beam; a plurality of second beam transport lines (5A-D) branching from the first beam transport line and connected to the respective irradiation nozzles (15A-D; 15E-H); a plurality of switching magnets (6A-C) disposed on the first beam transport line and adapted to selectively introduce the charged particle beam extracted from the charged particle beam generator into one of the second beam transport lines (5A-D); a beam dump device including a steering magnet that is disposed on the first beam transport line and a beam dump that is disposed on a beam path branching from the steering magnet, said beam dump device being adapted to control the excitation of the steering magnet such that from among charged particle beams extracted from the charged particle beam generator, a charged particle beam which is not to be transported to one of the plurality of irradiation nozzles (15A-D; 15E-H) is applied to the beam dump; and a dose monitoring device disposed in the beam dump device and adapted to measure the dose of the charged particle beam applied to the beam dump, characterized in that said charged particle beam generator further includes a voltage regulator adapted to control a radiofrequency voltage to be supplied to the radiofrequency-wave applicator on the basis of a predetermined gain pattern of the radiofrequency voltage and for controlling the intensity of a charged particle beam extracted from the synchrotron to the first beam transport line, and the charged particle beam irradiation system further comprises a voltage gain pattern creation device (30, 200b) adapted to create a voltage gain pattern to be set for the voltage regulator based on a measured value by the dose monitoring device.