PURPOSE: To set a prescribed oscillation frequency with a high precision by fine adjustment, by providing a conductive regulating bar electrically connected to a housing on a dielectric resonator and controlling the distance between the regulating bar and the dielectric resonator.

CONSTITUTION: A regulating screw 15 is moved upward or downward to change a distance (h) between this screw 15 and a dielectric resonator 6, thereby controlling the oscillation frequency of a microwave oscillation circuit. The oscillation frequency is raised according as the distance (h) is shortened, and the regulating screw 15 is set on the resonator 6 to which positive feedback is applied most intensively. A regulating screw 17 for fine adjustment of the oscillation frequency has conductivity and is engaged with an art above a position (p) of a substrate in the periphery of the screw 15. Since the screw 17 is slightly deviated from the position where positive feedback is applied most intensively, the screw 17 has less influence upon positive feedback than the screw 16. Consequently, the variation of oscillation frequency due to vertical movement of the screw 17 is small and the oscillation frequency is finely controlled by regulation of the screw 15.