To increase the rigidity of a lightweight panel without requiring plate thickness in an originally required extent even when a heavy article is fastened.

A long-sized fastening auxiliary member 4 is installed so as to be placed extending over the other end section from one end section of light-gauge steel 1 composed of a steel material for a substrate disposed on the rear of the panel for building, a fastening material 3 is inserted and fixed extending over the fastening auxiliary member 4 from the light-gauge steel 1, and the fastening auxiliary member 4 can be detachably mounted on the light-gauge steel 1 by operating the fastening material 3.