PURPOSE: To provide a thin oscillator which prevents an oscillation frequency from being flucturated by a vibration or a change with the lapse of time and holds air tightness in the oscillator by constituting an oscillation frequency control screw without fitting it to a metallic cap.

CONSTITUTION: The width of a pedestal part 2 vertical to the lengthwise direction of a metallic cap 6 is made almost equal to the width of the metallic cap 6. Thus, right and left positions can be fixed only by fitting the metal cap 6 to a metallic base 1. On the other hand, the width of the metal cap 6 in the lengthwise direction of the metallic cap 6 is made wider than the width of the pedestal 2. Thus, the metallic cap 6 can be slid to right and left, and the position can be controlled. On the inner face of the metallic cap 6 constituting the microwave oscillator, a cut-out metallic piece 9 provided just above a dielectric resonator 5. When the metallic cap 6 is slid before performing welding and sealing, the metallic piece 9 is similarly moved, as well and a distance between the metallic piece 9 and the dielectric resonator 5 is changed so that the oscillation frequency can be controlled.