A rubber plug for closing each service hole has a cylindrical seal surface to be fitted in the service hole, and a flange to be brought into contact with an outer surface of the housing. The rubber plug has a cylindrical cup-shaped cored bar inside. The cored bar has a disc-shaped flange-portion cored bar, outside the housing, which is greater in a radial direction than a cylindrical portion of the service hole. The cored bar has a convex portion in a cylindrical-cup bottom of a recess, of the rubber plug, which is formed inward of the housing. The convex portion is coaxial with the cylindrical seal surface and projects toward an outward direction of the housing. A lip seal to be brought into contact with the housing is provided at a surface of the flange.