A portion of an insulation member makes contact with a surface of a lid body by receiving a caulking force from a caulking member either directly or indirectly via another member. When not receiving the caulking force, a surface of the portion of the insulation member facing the lid body becomes a curved surface which indicates a curve bulging in a load direction of a caulking load on a cut plane, the cut plane cutting the portion of the insulation member in a longitudinal direction of the lid body. When kept in the fastened state by receiving the caulking force, the portion of the insulation member is elastically deformed and the surface of the portion of the insulation member facing the lid body is brought into close contact with the surface of the lid body.