Abstract: The invention relates to a radio-frequency particle accelerator. First and second cylindrical inner conductors 4 and 5 are disposed on the axis of the particle beams from the particle beam entrance with an accelerating gap interposed between the inner conductors in a TM or TEM mode particle accelerating cavity. An end of the first inner conductor 4 and an end of the second inner conductor 5 are joined to base plates of an outer conductor 3 of the accelerating cavity to form an inductance and together with the capacitance across the gap to form a resonant cavity. In order to synchronize particle beams with the radio-frequency accelerating phases, a bunching gap 11 with an inductance is formed by forming slots 11a on the first inner conductor 4. Thus, a radio-frequency electric power for exciting the accelerating cavity 2 is automatically supplied to the bunching gap 11 through the inductive coupling.