A particle monitoring apparatus 26 having a dose monitoring device 26 for measuring a dose of a particle beam and a small-sized ionization chamber 38 which is smaller than the dose monitoring device 26 for measuring a dose of a particle beam passing through the dose monitoring device 26, and irradiating a particle beam to the irradiation position of the irradiation object by scanning the particle beam The radiation therapy apparatus measures the dose of the particle beam irradiated by the dose monitoring device 26 and measures the dose of the particle beam passing through the dose monitoring device 26 by the miniature ionization chamber 38 to measure the dose of the particle beam measured in the miniature ionization chamber A correction coefficient of the dose measured by the dose monitoring device 26 corresponding to the irradiation position is determined.粒子線の線量を測定する線量監視装置26と、前記線量監視装置26を通過する粒子線の線量を測定する前記線量監視装置26より小型の電離箱38とを有し、粒子線を走査して照射対象の照射位置に粒子線を照射する粒子線治療装置であって、前記線量監視装置26で照射された粒子線の線量を測定し、前記線量監視装置26を通過する粒子線の線量を前記小型の電離箱38で測定して、前記小型の電離箱38で測定した粒子線の線量を基に、前記照射位置に対応する前記線量監視装置26で測定された線量の補正係数を求める。
