There is a gap between the respective inner regions of the end face of the cylindrical side wall and the abutting portion of the top plate, which are situated inside the seal member. Even though the top plate is bent inward by atmospheric pressure when the container is decompressed to a predetermined degree of vacuum, therefore, the abutting portion thereof cannot come into contact with the inner edge of the end face of the cylindrical side wall. Thus, if decompression and exposure to atmospheric pressure are repeated to bend the cylindrical side wall repeatedly, there is no possibility of the inner edge portion of the end face of the cylindrical side wall being separated or rubbed off to produce dust. Moreover, the cylindrical side wall and top plate are joined together in the region outside the seal member. If dust is produced by the contact between the cylindrical side wall and top plate at their junctions outside the seal member, therefore, it is prevented from entering the container by the seal member. Thus, a clean atmosphere in the container cannot be contaminated by the dust.