A particle therapy system in which the efficiency of extracting a beam from a synchrotron can be improved and time required for therapy can be shortened is provided. The synchrotron 10 of the particle therapy system 100 extracts a charged particle beam, which circulates in the synchrotron 10, out of the synchrotron 10 by means of a slow extraction method using the resonance of a betatron oscillation, and magnetic poles 73 included in a bending magnet 12 of the synchrotron 10 have a SIM structure that generates a magnetic field distribution that makes the horizontal tune of the charged particle more closely approach a resonant line used in the slow extraction method as the amplitude of the horizontal betatron oscillation of a charged particle included in the charged particle beam becomes larger.