[Problem]A particle therapy system capable of reducing the installation area and also suppressing a variation in the irradiation beam position is provided.[Means for Resolution]A synchrotron 10 which generates a charged particle beam, a beam delivery system 40 which irradiates an irradiation target 51 with a charged particle beam extracted from the synchrotron 10 thereby forming a radiation field, a rotating gantry 30 which is provided with the beam delivery system 40 and is rotatable around the irradiation target 51, and dispersion measuring devices 42a and 42b, each of which measures the dispersion of the charged particle beam at the position of the irradiation target 51 at a plurality of rotation angles of the rotating gantry 30, are included, and it is characterized in that the orbit center of the charged particle beam extracted from the synchrotron 10 and the rotation axis 33 of the rotating gantry 30 are located on substantially the same straight line.
