An elastic seal member is formed at the surface of a metal plate included in a gasket, which faces toward a lid, and another elastic seal member is formed at the surface of the metal plate facing toward a side wall. At the metal plate, the seal member is disposed further inward relative to the other seal member fitted in a seal groove. As bolts are tightened, the seal members become compressed and deformed, causing the seal member to press the metal plate against an exposed portion of an inner wall portion and the other seal member to press the metal plate against the lid.