To prevent the occurrence of scalelike eruption flaw at the external surface by carrying out soaking treatment under the condition that the soaking temperature and soaking time of a billet satisfy a specific relation.

Billet is composed of a duplex stainless steel containing 1.5-5.0 wt.% W. In a soaking treatment of the billet, the soaking temperature and soaking time satisfy the following inequality: 300≥(t/400)×{-(T-1200)2+40,000}, where (t) is soaking time (hr) and T is soaking temperature (°C). Further, T is made to 1,000 to 1,400, preferably to 1,100 to 1,320. A steel tube manufactured by the method is required to have seawater corrosion resistance in particular when used for a submarine flow line, and therefore it is preferable that the billet contains, other than W, prescribed amounts of C, Si, Mn, P, S, sol.Al, Ni, Cr, Mo, and N and also contains prescribed amounts of one or more elements among Ca, Mg, REM, and B and/or one or more elements among Cu, V, Ti, and Nb.