A charged-particle beam accelerator includes an RF-KO unit for increasing the amplitude of betatron oscillation of a charged-particle beam within a stable region of resonance and an extraction quadrupole electromagnet unit for varying the stable region of resonance. The RF-KO unit is operated within a frequency range in which the circulating beam does not go beyond a boundary of the stable region of resonance, and the extraction quadrupole electromagnet unit is operated with appropriate timing as required for beam extraction so that the charged-particle beam is extracted with desired timing.