To join a frame with a filter and to integrate them into one body to improve sealability and durability by setting a film a prescribed shape in a cavity, next feeding unvulcanized rubber into a cavity around a filter, then heating the unvulcanized rubber to decompose and vulcanize a foaming agent, and making a part thereof intrude into the filter.

The plane shape of a filter 1 corresponds to that of a filter body shell. When forming a frame 2, unvulcanized rubber is heated to decompose and vulcanize a foaming agent at the same time to make a part 34 there intrude into the permeable filter 1 and enable the joining with and the integration into the filter 1. Since the frame 2 is made of sponge rubber, it is rich in elasticity and exerts seal force. The frame 2 made of sponge rubber is formed into a frame by performing vulcanizing formation in a metal mold. Therefore, there appears no bubble 32 on the surface of the frame, and the surface thereof is covered with unfoamed surface rubber matter 31, and bubbles 32 are present only inside. Therefore, there is no danger of gas being slipped away and the frame being crushed.