PURPOSE: To adjust the gap in the axial direction constant easily by tapering the outer face of a movable member and the inner face of a ring facing each other thereby providing a micro conical gap between them.

CONSTITUTION: When fixing a movable member 18a to an anode support 11a through a bolt 12, a spacer 21 is inserted between said support 11a and member 18a to adjust the gap (g') in the axial direction between said member 18a and a ring 19a. When properly selecting the thickness of the spacer 21 in correspondense to the dimensional fluctuation of anode support 11a, movable member 18a and ring 19a, the gap (g') having predetermined dimension can be obtained while the vibration absorbing performance can be easily adjusted. A vibration damper 20' is constructed by utilizing the fluidity of insulating oil 15 in a minute conical gap (g') constructed with the movable member 18a having tapered outer face and the ring 19a having tapered inner face.