A resin sub-seal member (11) provided with a concave groove (16) is disposed at a region closer to a higher-pressure side region than a rubber main seal member (10) and pressure variation in a gas on a higher-pressure side is inhibited from being transmitted to the main seal member (10). Further, a variation reducing space (13) is formed between the seal members (10) and (11) to be able to reduce the pressure variation when gas leakage in the sub-seal member (11) occurs or the sub-seal member (11) responds with a delay with respect to the pressure variation. Thus, rapid change in the pressure of the gas surrounding the main seal member (10) is inhibited, occurrence of the blistering phenomenon is inhibited, and a high seal ability is achieved. The seal structure is formed by a simple structure including the seal members (10) and (11), and the variation reducing space (13).