A particle beam therapy system is disclosed which includes: a synchrotron 200 accelerating a charged particle beam injected from a pre-accelerator 11 up to a predetermined energy level before applying a high-frequency voltage to an extraction device to extract the charged particle beam caused to exceed a stability limit； a beam transportation system 300 transporting the charged particle beam extracted from the synchrotron 200 up to a treatment room 400, and an irradiation device 500 irradiating a patient 41 in the treatment room 400 with the charged particle beam in conformity to the patient's tumor shape. The synchrotron 200 has functionality to accelerate or decelerate the charged particle beam successively to extract the charged particle beam at a plurality of energy levels during an extraction phase of the synchrotron 200, the beam transportation system 300 further having functionality to block off an unnecessary charged particle beam extracted from the synchrotron 200 during acceleration or deceleration.
