PURPOSE: To realize the same effect as a metallic case inexpensively by covering a board with a cover whose inner face is made of a metallized insulator in a dielectric resonator loading microwave oscillator.

CONSTITUTION: A semiconductor element 3 and a dielectric resonator 4 forming the microwave oscillator are mounted on the board 2. The board 2 is covered by the cover whose inner face is made of the insulator 5 provided with a metallized layer 6. Through the structure above, a magnetic field 10 forms a closed loop in the case the same as being surrounded by a metallic case and an output is obtained at an output terminal 7. Thus, the same effect as the provision of the metallic case is realized inexpensively.