To provide a structure, a method and a mounting metal fitting for mounting a roof and a floor panel, which enable welding fixation of the mounting metal fitting after laying the ALC panel, which can reduce a risk of high-place work by dispensing with a scaffold for the welding work, and which can enhance working efficiency.

The mounting metal fitting for use has a flat bottom, and can be stably placed in a self-supporting state on a beam. After the ALC panel is horizontally laid and arranged on the beam, the mounting metal fitting is welded and fixed to the beam of a building frame. Then, the ALC panel is laid again, and the foregoing operations are sequentially repeated. Preferably, the mounting metal fitting is made of a flat steel plate with a thickness of 8-20 mm and a length of 30 mm longer, while the ratio between the height and the thickness is 3 or thinner. In addition, at least one of a nut portion, a bolt portion and a through-hole can also be included.