PURPOSE: To reduce the change of the oscillated frequency due to a change of humidity, by packing a foaming plastic agent into a metallic case which encloses the resonator of a dielectric resonator control oscillator.

CONSTITUTION: A foaming plastic agent 1 is packed into a metallic case 2 which encloses dielectric resonator 3. The oscillated frequency varies to a change of humidity or pressure owing to the change of dielectric constant of the air enclosing the resonator 3. In this connection, the air enclosing the resonator 3 is replaced with the foaming plastics. Therefore the moisture does not easily get into the part packed with the foaming plastics although the humidity of the open air changes. Thus the change of the environmental humidity enclosing close around the resonator 3 is reduced. This reduces also the change of the oscillated frequency.