To prevent a curvature of a reinforcing wire net when the reinforcing wire net receives power generated by a flowage of a slurry in a mold form and an expansion function in foaming, and directed to the outside of the mold form at a step of holding the reinforcing wire net in the mold form to inject the raw material slurry of autoclaved lightweight concrete in the mold form to foam and cure.

The raw material concrete slurry is injected into the mold form in a state that the reinforcing wire net is held in the mold form, and a tension is given in a panel length direction of the reinforcing wire net, foamed and cured. In order to give the tension in the panel length direction of the reinforcing wire net, 90 of rotation of a support pin supports the reinforcing wire net, and at the same time, a projection for tension development is provided on the support pin, pressing a wire rod of the reinforcing wire net to spread the wire net, which generates the tension.