PURPOSE: To prevent lateral vibration by supporting a movable shaft for moving a tuner cup through a roller bearing structure at the intermediate section while supporting the outer end section movably through a holding structure containing a rotary screw shaft and a coupling board.

CONSTITUTION: Charged particles are passed into a hollow tube 23 in the center of an accelerating resonance cavity of a particle accelerator to feed microwave into the cavity while a tuner cup 28 in a tuner 27 held under super high vacuum is moved by a bellows 35 to correct the resonance frequency. Here the movable shaft 29 of the cup 28 is supported by a roller bearing structure 80 at the intermediate section while the outer end section is supported by a holding mechanism 85 comprising a rotary screw shaft 64, guide rod 68, slidable nut 66 and a coupling board 67 to be held by them then the screw shaft 64 is rotated by a motor 59 to move the shaft 29. Consequently when moving the cup 28, lateral vibration will not occur to enable highly accurate changing of cavity characteristic.