To use a coupler housing chamber as a reinforcing beam between flexible plate faces so as to bring the coupler housing chamber side wall into contact with a coupler for preventing the coupler from turning by expanding the recessed cavity type coupler housing chamber formed in a first plate face part to a second plate face part in formation.

A coupler 14 having a lead wire 14b is housed in a coupler housing chamber 13 formed in a bracket. The coupler housing chamber 13 is formed into a recessed cavity shape in a first plate face part while expanded toward a second plate face part 11b. As the coupler 14 is installed with fitted inside the coupler housing chamber 13, the coupler 14 is prevented from protruding and its mounting structure can be compact in size. Because the coupler housing chamber 13 is arranged in a dead space, a space is formed between an interior plate and it, and interference can be prevented. The coupler housing chamber 13 functions as a reinforcing rib connecting flexible inside face parts of the first and second plate face parts 11a, 11b together. As two sides are opened in the recessed cavity type coupler housing chamber 13, assembly of the coupler 14 is not disturbed.