A tool insertion hole (25) includes a pair of guide faces (71, 72) facing each other while being parallel with a widthwise direction of a plate spring (34), a separation distance between the guide faces (71, 72) becoming narrower toward the back side, in which the pair of guide faces are adapted to, by the time a tool (61) reaches a position to release an engagement, contact with each of end faces (62), at least one of the guide faces being adapted to make face-contact with one of the end faces (62) or make line-contact with the one of the end faces along a widthwise direction.