PURPOSE: To facilitate the positioning set work between a connecting member and a hollow pillar member and improve the welding quality by forming hollow pillar member connecting tube sections located with end faces at a preset distance to the outside of diaphragms from the diaphragms on the tubular plate material of a connecting member.

CONSTITUTION: Hollow pillar member connecting tube sections 12, 13 located with end faces 6B, 6C to the outside of diaphragms 10, 11 from the diaphragms 10, 11 are formed on the tubular plate material 6 of a connecting member 3. Bevels are formed on the end faces 6B, 6C of the connecting tube sections 12, 13. The connecting member 3 and hollow pillar members 1, 1 are set on a pair of welding rotary jigs while an upper back plate 8 and a lower back plate 9 are inserted into the lower end opening 1A and the upper end opening 1B of the hollow pillar member 1 respectively. A welded connection section W1 made of a deposited metal is buried between the end face 1C of the hollow pillar member 1 and the bevel, a welded connection section W2 is buried between the end face ID and the bevel, and the hollow pillar members 1, 1 are connected to the connecting tube sections 12, 13 by robot welding to complete an assembly pillar 14.