Holder 71 by abutting on the triangular protrusion 72e of the operating member 72 when moving the operating member 72 in a predetermined direction, the stopper 71e to restrict the movement of more than a predetermined amount in a predetermined direction of the operation member 72 comprises. Operating member 72, when in one position of the locked and unlocked positions, when a force in the direction perpendicular to the predetermined direction in the operation protrusion 72a, a direction in which the flat portion 72c perpendicular deflection, beyond triangular protrusion 72e is a stopper 71e, so as to be movable between a locked position and an unlocked position, and is formed of an elastic material.