To prevent the possibility of catching a finger by mounting a fitting piece in a recessed part formed on each of two metal fittings so as to be rotatable within the recessed part, and vertically changing the direction of an arm mounted on each fitting piece only by simply sliding it as is in the fitted state by the rotation of the fitting piece.

Recessed parts 1a are formed symmetrical in two positions on the inside of a metal fitting 1 so as to protrude outward. A small recessed part 1b is further provided in the recessed part 1a, and a small protruding part 1h is formed integrally in the other position. A fitting piece is inserted to the right recessed part 1a of each metal fitting, so as to be rotatable within the recessed part 1a with the fitting part as a fulcrum. The upper arm hole of two mutually fitted arms is inserted and fitted to the protruding part 1h of the fitting piece, and the lower arm hole is fitted to the protruding part 1h. Thereafter, two metal fittings 1 are connected by inserting and fixing a screw 8. When the arms are moved vertically, only one of the fitted arms is slid by the rotation of the fitting piece.