PURPOSE: To enhance the toughness of the welded part of an seam welded steel pipe and to improve the low temp. toughness of said steel pipe, by heating the seam welded steel pipe subjected to high frequency welding to a specific temp. higher than an annealing temp., and cooling the heated steel pipe before annealing.

CONSTITUTION: An seam welded steel pipe subjected to high frequency annealing is heated to about 1,000W1,300°C exceeding an annealing temp. After cooling, the steel pipe is heated to an annealing temp. of (Ac3 point+40W60°C) to refine the roughened structure of the welded part. By this method, the toughness of the seam welded steel pipe is enhanced while the low temp. toughness thereof is also improved and said seam welded steel pipe is adapted to a use temp. condition over a wide range to braden the application range thereof.