A thin plate member which has a groove portion recessed toward the inside in a radial direction with a gap interposed between the thin plate member and the outer surface of a site on the front end side of a main body section. At least one end of the thin plate member is joined to the outer surface of the main body section and is provided radially outside the site on the front end side of the main body section. A seal member is disposed in the groove portion and has elasticity due to resin. Since the gap functions as a heat-insulating layer and is interposed between the seal member and the main body section, heat conduction from the main body section, that is, heat conduction through a first pathway is reduced, and thus deterioration of the seal member due to heat is suppressed.