PURPOSE: To prevent the Q of a resonance circuit at no load even if a resonance frequency is changed over a wide range by constituting a part of a resonance frequency variable member facing at least a dielectric resonator by a dielectric.

CONSTITUTION: The resonator consists of a metallic chassis 1, a metallic cover 2, a dielectric resonator 3, a supporting base 4, a connector 5 for input/output, and an antenna 7. In changing a resonance frequency, a metallic screw 8 fitted to the cover 2 is adjusted. The dielectric 9 is provided to the part of the screw 8 facing the resonator 3 and the dielectric 9 is coupled with an electromagnetic field being leaked from the resonator 3 in approaching the dielectric 9 to the resonator 3, then the equivalent size of resonator is decreased and the resonance frequency is lowered. When the loss of the dielectric 9 is made equal to that of the dielectric of the resonator 3, the loss is almost negligible and the Q of the resonator at no load is not lowered.