To facilitate application of a robot system by enabling welding with a flat joint in producing a steel frame column for construction.

An outer diaphragm or through diaphragm 5, which has an external profile roughly symmetrical to a cross sectional shape of a steel frame stock, is arranged at a prescribed position of a square steel tube steel frame 1, a beam connection member 6 is arranged from the side facing the vertical part of the outer diaphragm or through diaphragm 5, to a flat joint part where each tip edge part of the outer diaphragm or through diaphragm 5 and the beam connection member 6 are faced, vertical upward welding is conducted for the groove in the vertical direction formed with using a backing 9 as a welding nozzle 10 is erected from underside to upperside. As compared to the welding with using a conventional T joint, vertical upward welding makes penetration of fused metal easy, reduces a number of weld passes and makes robotization of a welding machine easy.