PURPOSE: To suppress the generation of a welding defect caused by irregular welding and to prevent an erroneous welding from being generated by melting and welding the edge parts of both facing steel strips respectively at a low pressure welding rate.

CONSTITUTION: Steel strips H, H are transported respectively between breakdown rolls 3, 3... and bent in symmetry to the line and they are bent symmetrically, sequentially by facing surfaces of fin pass rolls 5, 5... so that the respective sections in the width direction are made into a semicircular shape. The welded end faces of the steel strips H, H are preheated by preheating high frequency welding machines 7', 7', the steel strips H, H are sent to TIG welding machines 7, 7, both edge parts of them are melted and welded and integrated by a squeeze roll 6 by adding a side pressure in such a range that forming is not unstable. Accordingly, the welding defect caused by the irregular pressure welding rate is suppressed and weld of the edge pats is conducted securely and smoothly and errors in the welded part can be prevented from being generated.