PURPOSE: To dispense with an additional mechanism for changing a resonance frequency, in a RFQ accelerator for supplying a high frequency power to the opposed faces of a quadrupole electrode forming a sine functional waveform to accelerate an ion beam, by allowing the continuous change of the final energy without changing the resonance frequency.

CONSTITUTION: A sine functional waveform is generated from the quadrupole electrode of a RFQ accelerator, and a flat part having no waveform amplitude or a flat vane part 2 is inserted in an optional position of a vane 1 in which this waveform is formed. Thus, the final energy is changed according to the position where the flat vane 2 is inserted, and the energy of the ion beam can be optionally changed. Specifically, a dovetail groove 7 is provided on the bottom part 6 of each vane forming the flat vane part 2, and a dovetail tenon 8 to be fitted to the dovetail groove 7 is provided on the upper part 5 of the vane, and the upper part 5 is laterally slid. Thus, only the vane upper part 5 can be pulled out without pulling out the whole vane, and several kinds of upper parts 5 are prepared, whereby the final energy is changed.