To improve the workability by forming the angle part of a rectangular frame by use of a cutting piece, cutting out a floor panel angle part in a rectangular form by the cutting piece part to form a cutout part, and housing a column angle part in the cutout part to vertically juxtapose floor panels.

A plurality of columns 1 is arranged lengthwise and crosswise, a beam member 2 is erected between the vertically adjacent columns 1 to form a skeleton 3, and a floor panel 6 having plate members 5 mounted on both sides of a rectangular frame 4 is erected on the beam member 2. The angle part of the rectangular frame 4 is formed by use of a cutting piece 7, the angle part of the floor panel 6 is cut out in a rectangular form by the cutting piece 7 part to form a cutout part 8, and the angle part of the column 1 is housed in the cutout part 8 to vertically juxtapose the floor panels 6 without clearance. The rectangular plate-like cutting piece 7 is arranged in the position corresponding to the angle part of the rectangular frame 4 of the floor panel 6, a plurality of joists 10 is juxtaposed by nailing, and a square pole-like cleat 9 is erected by nailing. Accordingly, the supporting strength for the floor panel can be improved.