PURPOSE: To make it possible to adjust the frequency easily even when the resonance frequency of a cavity is deviated significantly by connecting the outer peripheral portion of an accelerating electrode to the inside portion of a side plate with a flaxible connector so as that the outside surface of the side plate and the outside end portion of the accelerating electrode is connected to be movable to adjust the length of the gap of the accelerating electrode.

CONSTITUTION: The inner side of a side plate 2 and the outer periphery of an accelerating electrode 3a is welded through a flexible connector 9 and connected vacuumtightly. The accelerating electrode 3a is made to be movable in axial direction by means of flexture of the flexible connector 9, and the other electrode 3b is welded and attached to the side plate 2 so as to be displaced. Therefore abnormal heat generation due to the increase of contact resistance can be prevented, and stable high frequency current can be flowed for the sake of vacuum seal of sliding portion even when the accelerating electrode is moved for adjusting the gap Δ. Thereby the gap Δ between the accelerating electrode 3a and 3b can be adjusted easily, and the substantial deviation of the resonance frequency caused by manufacturing error, deformation due to the temperature and the pressure or the additional compaction of a gasket 8 during the baking process can be adjusted within the control range of a tuner 5.