PURPOSE: To suppress a welding deformation and a dimension error to the minimum and also to improve a work efficiency by welding a core part and upper and lower pillar parts obtained by cutting a pillar material, by use of a diaphragm and a backing metal, and thereafter, executing the whole assembly and welding.

CONSTITUTION: Upper and lower pillar parts 3, 4 are obtained by cutting a core part 2 corresponding to a pillar beam joint part from a pillar material 1 made to prescribed length. Subsequently, a diaphragm 5 is fitted through a backing metal 6 so as to close up an opening of the core part 2, and also a backing metal 7 is fitted to the inside face of the side end part of the core part 2 of the pillar parts 3, 4. Next, the pillar parts 3, 4 are joined to the core part 2 so as to place the core part between them, and also a beam joint member 8 consisting of H-shape steel is fitted from four directions. In this state, a space part S formed by the end face of the respective flange plates P of the core part 2, the pillar parts 3, 4, the diaphragm 5 and the member 8 is welded by use of a copper slide strap 9.