The lock member includes: a plate-like base portion positioned on a top side of the socket; a holder retaining the base portion to the socket; an operating portion connected to the base portion and movable in a direction intersecting the base portion by an external operation; and a lock portion restricting disengagement of the engaged portions and the engaging portions. The lock portion is positioned in the socket and connected to the operating portion via a joint inserted in the cutout. Without the external operation, the lock portion is present in movement paths where the engaging portions move to be disengaged from the engaged portions. When the operating portion is moved by the external operation, the lock portion is moved away from the movement paths.