To facilitate transportation and assembly by providing fitting holes and cylindrical fitting holes at one end of an L-shaped angle forming frame, providing moving projections, insertion projections, and cylindrical projections at the other end, and providing protruded plates having insertion holes.

A fitting forming section 8 at the left end section of each of four L-shaped angle forming frames 2 is provided with multiple fitting holes 8a-8m, cylindrical fitting holes 10-10b, multiple moving projections 6a-6d, insertion projections 7-7h, and cylindrical projections 9-9b. Multiple protruded plates having insertion holes 19d are provided inside the angle forming frames 2. The moving projections 6a-6d and insertion projections 7-7h are fitted into the fitting holes 8a-8m of adjacent angle forming frames 2, and the cylindrical projections 9-9b are fitted into the cylindrical fitting holes 10-10b to form a column base frame. Opening preventing tools 20 made of an opening preventing band are inserted into the insertion holes 19d of the protruded plates of the angle forming frames 2. When the column base frame is to be enlarged, an extension frame is connected between the angle forming frames 2. Transportation and assembly can be facilitated.