The tip parts 626a and 626b of these insulating tubes 625a and 625b are molded to be of an outside diameter smaller than the outside diameter of the other parts of the insulating tubes 625a and 625b An insulating member 627 is internally fitted and secured by such means as bonding inside the insulating tube 625a at the rear end to secure watertightness Further, the insulating tubes 625a and 625b a