A contact point member comprises contact point pieces extending from both ends of a base portion and a gripping portion extending from the center of the base portion. The base portion is placed on an upper surface of a holding portion in a base member and is inserted between a main wall of a surrounding wall and stoppers on the holding portion for positioning. After a front end of the base portion is pressed and inserted under a presser portion provided in the main wall, when the gripping portion is pressed on the holding portion, an engagement piece formed in the rear end of the gripping portion is engaged to a corner hole formed in a projection of the holding portion, thus preventing the falling-down of the contact point member. The mounting operation of the contact point member is easily completed simply by pressing down the gripping portion.