An RF accelerating cavity includes an accelerating cavity unit and an inductance varying device having a magnetic member connected parallel to an acceleration electrode gap. The RF accelerating cavity is tuned in such a fashion that a charged particle beam acceleration frequency matches a resonant frequency of the RF accelerating cavity by regulating inductance of the inductance varying device in accordance with a changing pattern of the charged particle beam acceleration frequency. Alternatively, impedance of the RF accelerating cavity is increased with the provision of a fixed inductance connected parallel to the acceleration electrode gap when the RF accelerating cavity has a narrow acceleration frequency range.