PURPOSE: To simplify the structure of the cavity part and also facilitate the easy working for sealing, by inserting the air sac into the cavity part of the cavity resonator and then filling the gas substance into the air sac for sealing.

CONSTITUTION: Coupling window 3 of waveguide 2 plus hole 6 of the inner wall surface are drilled to cavity resonator 1, and then air sac 7 formed with the synthetic resin such as the polyethylene or the like into one body plus vent tube part 8 are inserted into cavity part 5 and hole 6 of resonator 1 each. The gas substance of the fixed pressure is enclosed into the resonator from part 8 of inserted sac 7 so that the gas substance may touch close to the inner wall of cavity part 5. Then part A of part 8 is heated up in the state under which the gas substance is encloded to secure the airtight sealing. Thus state 9 is secured. Thus the structure is simplified at part 5, at the same time facilitating the easy working for sealing.