To reduce time and labor required on the occasion of fitting a wall panel as a finishing material to a skeleton of a building and thereby to attain shortening of a term of works and reduction of the cost of construction.

On the occasion of fitting a wall panel to a skeleton of a building, hat type steel 10 is fitted in a plurality of stages in the lateral direction with the top side set on the skeleton side and thereby a hat type lateral body edge is provided. Then, the wall panel 20 prepared by backing a metal plate 21 having a groove-shaped section and a flange 23 with a backing material 22 is fitted by fixing the flange 23 to a flange part 12 of the hat type lateral body edge. Since the hat type steel 10 has the open section, it can be joined directly to the surface of the skeleton with ease and tightness by bolting and it is unnecessary to provide a stile for fixing the steel.