PURPOSE: To suppress the undesired resonance mode by providing a conductor member at a position where the electric field strength at the undesired resonance mode is maximum.

CONSTITUTION: A metallic rod 16 being a conductive member is projected in a case 11 at a position from the installed center of a dielectric resonator 13 apart by a distance of a diameter D of said resonator 13 and the quantity of projection is provided adjastable externally from the case 11. Even when the inner diameter of the case 11 is not suitable for the basic mode TEO1δ and might cause the undesired TE11δ mode, since the metallic rod 16 is arranged at a point where the electric field of the mode TEO1δ is maximum, the undesired mode TE11δ mode is suppressed. Further, the restriction of the size of the case is eliminated to increase the degree of freedom of the circuit design and the resonance at the stable basic mode is obtained without losing the economy.