To obtain an extra-low carbon steel tube having the hardness in the weld zone equal to that of a base metal part which the steel tube is produced.

A steel slab contg. ≤0.01% C, ≤0.05% Si, ≤0.3% Mn, ≤0.03% P, ≤0.02% S and ≤0.08% sol. Al, contg. one or two kinds of 0.002 to 0.10% Ti and 0.002 to 0.10% Nb, and the balance Fe with inevitable impurities is heated to the temp. range of 1100 to 1280°C and is thereafter subjected to hot rolling so as to regulate the finishing temp. to ≥ the Ar3 transformation point and the coiling temp. to ≥500°C, and the obtd. hot rolled coil or a cold rolle coil obtd. by subjecting the hot rolled coil to cold rolling is used as a stock, the 2 to 4 mm widths of both edge parts of an opening pipe cylindrical subjected to continuous forming by a forming roll group are preheated at 700 to 1000°C, are subjected to normal heating and are subject to electric resistance welding, and the weld zone is allowed to cool to obtain the hardness of the weld zone approximately equal to that of the base metal part while electric resistance-weld is performed, by which the heat treating cost can be reduced, and, moreover, the problem of scale exerting a bad influence on a die in bulging can be solved as well.