The button (5) is swingably supported by a button support member (11) with the help of a button support pin (13). A link member (7) is turnably connected to the button support member (11) with the help of a link support pin (23). When the button (5) is operated to be pressed, a detent rod (9) moves downward with the help of the link member (7). The button support member (11) is inserted into and disposed in a base member (33). An upper wall (35c) of the base member (33) is provided with a base member projection (43) including a fitting hole (43a). An upper wall (21) of the button support member (11) is provided with a support member projection (45) including a fitting recessed portion (45a). A fitting pin (47) is inserted through and fitted to the fitting hole (43a) and the fitting recessed portion (45a).