Abstract: The accelerator is a cyclic type accelerator having deflection electromagnets and four-pole electromagnets for making a charged particle beam circulate, a multi-pole electromagnet for generating a stability limit of resonance of betatron oscillation for the production of the charged particle beam, and a high frequency source for applying a high frequency electromagnetic field to the beam to move the beam to the outside of the stability limit, thus exciting resonance in the betatron oscillation. The high frequency source generates a sum signal of a plurality of AC signals of which the instantaneous frequencies change with respect to time, and of which the average values of the instantaneous frequencies with respect to time are different, and applies the sum signal via electrodes to the beam.