PURPOSE: To efficiently carry out production of stable quality in welding the insides of both ends of square cylindrical column base material by erecting the column base material alternately and performing welding from the upper surface.

CONSTITUTION: The column base material 31 is erected up to an almost vertical state with the one end side upward and a welding head of a welding robot 120 is inserted from the upper side thereof and the overall periphery of the diaphragm 34 is welded. Afterward, the column base material is brought down up to a horizontal state and continuously, conveyed ahead and then, the column base material 31 is erected up to the almost vertical state with the other end side thereof upward and the overall periphery of the diaphragm 34 is welded by the welding robot 120 from the upper side thereof. Since two welding robots perform continuously welding work, it is efficient and the stable welding quality can be obtained by welding from the upper side.