PURPOSE: To prevent a crack taking place on a welding end part of a steel pipe giving the bending deformation constraining forces to maintain constant the curvature of the steel pipe in specific regions.

CONSTITUTION: At the time of performing the seam welding in the pipe axial direction of the pipe 1, a clamper 9 type jig to hold a pipe end 4 is used. By this method, constraining points are restricted in the regions within 150mm respectively at both sides in the pipe peripheral direction to hold a weld line 5 between within 300mm toward the anti-welding direction from the welding end 4. Accordingly, in the regions, the bending deformation constraining forces to check the bending deformations 15 and 15a of the steel pipe 1 with the seam line 5 as rotation axis taking place by the seam welding thermal stress are given at the constraining points ≥2 points and when the curvature of the pipe 1 is maintained constant in the regions to perform the welding, the crack 3 does not take place and the steel pipe welding with the high efficiency and high quality can be performed.