The present invention improves the accuracy of therapy by checking in real time whether an spread-out Bragg peak (SOBP) width agrees with a desired width during irradiation with a beam. The device for outputting a charged particle beam includes a charged particle beam generator 1 including a synchrotron 4; a range modulation device such as a range modulation wheel (RMW) 28 which forms a Bragg peak of an ion beam extracted from this charged particle beam generator 1; an irradiation device 16 which is located in the direction of ion beam propagation of this RMW device 28 and includes a dose monitor 31 for detecting a dose of the ion beam; and an SOBP width calculation device 73 which calculates ion beam Bragg peak formed by the RMW device 28 based on a detection value of the dose monitor 31.