To stably manufacture electric-resistance-welded steel pipes with superior roundness without reducing the manufacturing efficiency with necessary minimum rolls by reducing the weld zone with horizontal rolls immediately after heating/softening the weld zone of the welded steel pipe with high-frequency induction heating equipment after welding.

A material steel band 1 is fed to a forming roll group 2 having an edge forming groove and continuously formed in a cylindrical shape, and both edges are butted and welded with an electric resistance welder 3 while supported with squeeze rolls 4. The pipe 5 thus continuously welded is then heated with a high-frequency induction heating equipment 6, and the heated/ softened weld zone is immediately formed in a concentrated way using horizontal rolls 14. After passing through a water-cooling zone 7, it is dimensionally adjusted with a sizing roll group 8, then cut to a specified length with a traveling cutter 9, and subjected to surface inspection as a final product 10.