To improve the uniformity of mechanical property and the toughness by using a hot rolling steel of low Mn of a specific composition as the raw material, forming a raw tube with cold forming and welding, then hot-forming to a square tube at the time of producing a large square steel tube of square body or rectangular shape as a building material.

At the time of producing a square column like 350 to 400mm square × 12 to 22mm thickness, as a raw material is used of a steel plate of containing, by weight ratio, 0.12 to 0.18% C, 0.10 to 0.55% Si, 0.65 to 1.50% Mn, ≤0.02% P, ≤0.015% S, the remainder is composed of Fe, B and inevitable impurity, further the composition is ≤0.36% Ceq, ≤0.29% Pcm, 17 to 26% RA, and it is formed in the temperature of Ar3 transformation point. Further, for producing a tube, a steel plate 1 is made into a circular tube 4 by cold forming and welding, after then, it is made into a polygonal tube (raw tube) 5 by a polygonal tube forming mill. After then, it is preferably heated, formed with hot drawing with a fore stage and rear stage polygonal tube forming mills 28, 30 and a prescribed shaped square column tube 6 is made.