PURPOSE: To make it possible to control so that the oscillation characteristic of a competing mode is suppressed and that the oscillation of a main mode is generated in a stabilized manner by changing reflection phase from a resonance cavity of a dielectric plate.

CONSTITUTION: An axial direction slide section 10 is disposed in the vicinity of an output window which slide section is sealed airtightly with a diaphragm 7 capable of changing the position of a dielectric plate 1 in a tube axis direction. These components are connected and half-fixed with flanges 4, 6 and a distance adjustment rod 8. Even in the case the oscillation of a main mode is suppressed or the main mode oscillation is hard to occur, the reflection phase from the resonance cavity of a dielectric plate 1 is changed by controlling the position of the dielectric plate 1 in the tube axis direction. Thus the oscillation of the main mode is easily started in a stabilized manner.