PURPOSE: To easily check and correct the occurrence of a forming defect in pipe stock by mounting a shielding cover covering only the local part around a weld zone to lower part of the lifting section of a welding device.

CONSTITUTION: An electric resistance welded tube 1 is continuously produced by heating both edges at the seam of the pipe stock 1A by high-frequency energization from a contactor (contact chip) 11 of the high-frequency resistance welding device 10, upset welding both heated wedges by a squeezing roll 12. At this time, the shielding device 20 is installed around the weld zone of the pipe stock 1A to provide shielding around the weld zone with a shielding gas blown out of a shielding gas blow-off nozzle. The shielding cover 22 covering only the local part around the weld zone is mounted at the lower part of the lifting section 13 of the welding device 10. As a result, the hermetic space around the weld zone is formed to the min. volume, by which the concn. of the oxygen around the weld zone is lowered in a short period of time.