A charged particle accelerator capable of accelerating arbitrarily charged particles to an arbitrary energy level and resonating at a low frequency suitable for accelerating heavy ions, including quadruple electrodes which are supplied with high frequency power and disposed in the direction of the center axis of a cylinder-shaped container and a resonant circuit having a capacitor and an inductor for supplying a voltage to the quadruple electrodes. The capacitor is composed of a plurality of metallic plates provided along the center axis at specified intervals in the vicinity of the quadruple electrodes, and a plurality of conductive supports supporting the metallic plates which are directly connected to the container together with the supports and the container form the inductor. Since the metallic plates and the quadruple electrodes are electrically directly connected to each other, an arbitrary resonant frequency can be obtained by adjusting the intervals between the plurality of metallic plates with a position adjusting mechanism. In one embodiment, flat electrodes are protruded from opposite sides of the inner wall of the container and are disposed in parallel to the center axis and close to each other to constitute a capacitor, which makes it possible to have a resonant frequency in a low frequency range. To obtain a large Q value, the surface current resistance is lowered by covering the inner wall of the container and the surfaces of the flat plate electrodes with a superconductive material.