PURPOSE: To enable the formation of a groove for multi-layered building-up with decreased man-hours without requiring skill by providing the 1st and 2nd groove parts having prescribed sizes and shapes to the outside surface of steel pipe stocks and the 3rd groove part to the inside surface and cutting the bottom of the 1st groove part at a prescribed radius.

CONSTITUTION: The steel pipe stock is formed by cutting both edges of a steel plate for pipe making to the prescribed shape and size and subjecting the same to UOE working. Such stocks are butted and the 1st groove part 20 and 2nd groove part 30 having the prescribed sizes and shapes are formed to the outside surface of the stocks. The 3rd groove part 40 is formed to the inside surface of the stocks. The 2nd groove part 30 is tack-welded and after the 3rd groove part 40 is welded, the bottom of the 1st groove part 20 is cut at the prescribed radius R. The formation of such groove part does not involve the grinding as in the conventional practice to require a high degree of skill and shortens the working time to about 1/3; in addition, the weld defects are decreased to about≤1/5.