To assemble and execute a garage easily, and to thin the thickness size of a wall.

In the building such as a garage, U-shaped sectional mounting pieces 33 are welded and fixed onto the side sections of a vertical column 14 and an auxiliary vertical column, and an internal plate section 38 is directed towards the indoor side and an external opening section 41 is directed towards the outdoor side in the mounting piece 33. End sections are inserted into each mounting piece 33 from the external opening sections 41 and fixed by bolts 47 in horizontal columns 21 for reinforcement, and the vertical columns 14 and the auxiliary vertical columns are connected and fastened mutually. A wallboard is fixed onto the external surfaces of the horizontal columns 21 by using screws.