PURPOSE: To reduce the assembling cost of an L-shaped frame by providing a rigid coupling section at a nearly triangular arrangement with respect to hook sections.

CONSTITUTION: Hook pieces 2, 2' with claws 3, 3' are provided on the side face 10 of the right butt section 5 of an L-shaped frame 11 made of elastic plastic. A rigid center coupling piece 4 is protruded on the rear face at the middle of the hook pieces 2, 2'. These parts are arranged into an equilateral triangle with a vertex 4 as viewed from the direction A. Four L-shaped frames are connected in sequence into a square frame and coupled at butt sections 5, 6, coupling parts are positioned, and the claws 3, 3' are elastically hooked on hook holes 8, 8'. The frame assembling workability is improved, and the gap of butt faces and the release of hook claws 3, 3' are prevented.