PURPOSE: To prevent the welding defect due to oxidation from being generated in a welding part, by applying preliminarily carbon, an organic compound, or an organic silicide to the welding part of an open pipe to weld it when a welded steel pipe is produced by high frequency welding.

CONSTITUTION: A steel pipe ST is molded to an open pipe OP by many molding roll groups, and side end faces facing to each other of the pipe OP are heated by an induction heating coil 1 to which the high frequency current is flowed from a high frequency power source 2, and the pipe OP is pressurized by squeeze rolls and is welded into a welded pipe P. In this case, carbon of tar or the like, various synthetic resins, an organic solvent, rubber, an organic compound containing a high polymer compound such as cellulose, or an organic silicide such as a silicone resin is applied to side end faces to be welded. These applied materials are vaporized quickly to seal the part near the welding part from air by the vapor pressure, thus preventing the welding defect due to oxidation in the welding part.