PURPOSE: To obtain a thin microwave oscillator whose oscillation frequency can be controlled, by providing a conductive member freely expanded and contracted in the vertical direction on a dielectric resonator and providing freely movable conductive bars on the right and left of a housing and regulating the conductive member with conductive bars by expansion and contraction.

CONSTITUTION: An elastic annular conductive member 17 is placed just above a dielectric resonator 6 and is fixed there. Metallic screws 19 and 20 are engaged from both side walls 14c and 14d of the housing and have front ends 19a and 20a brought into contact with both side ends 17b and 17c of the member 17. When screws 19 and 20 are screwed inward, the member 17 is pressed by both ends 17b and 17c and is deformed, and a lower end 17d of the member 17 falls to shorten a distance (h) between the member 17 and the resonator 6. The member 17 is above the resonator 6 to which positive feedback is applied most intensively and the distance (h) has a great influence upon setting of the oscillation frequency. That is, the oscillation frequency is raised according as the distance (h) is shortened. Consequently, the oscillation frequency is controlled by screws 19 and 20.