In a particle beam therapy system which scans a particle beam and irradiates the particle beam to an irradiation position of an irradiation subject and has a dose monitoring device for measuring a dose of the particle beam and an ionization chamber smaller than the dose monitoring device, the ionization chamber measuring a dose of a particle beam passing through the dose monitoring device, the dose of the particle beam irradiated by the dose monitoring device is measured; the dose of the particle beam passing through the dose monitoring device is measured by the small ionization chamber; and a correction coefficient of the dose measured by the dose monitoring device corresponding to the irradiation position is found based on the dose of the particle beam measured by the small ionization chamber.