To prevent a operating part from being deformed by the impact of drop, etc.

First and second coupling members 2, 3 are folded in two and laid on top of a cover member 1, and under this state the second coupling member 3 is releasably locked to the cover member 1. In that case, the end 11a of the operating part 11 of the first coupling member 2 used in the operation of locking and unlocking the second coupling member 3 is not projected from the outer surface of the sidewall 5 of the cover member 1 but placed within a recess part 8 formed in the sidewall 5. Therefore, the end 11a of the operating part 11 can be protected by the side of the cover member 1, and therefore the possibility that the end 11a of the operating part 11 collides against objects because of drop is reduced, preventing deformation of the operating part 11 and enabling operating part 11 to operate smoothly at any times. Thus, even if the operating part 11 is exerted with the impact of drop, etc., the operation of clocking the second coupling member 3 can be performed well.