PURPOSE: To provide a seam welded steel pipe for mechanical structure, which is excellent in three point bending strength, and is used as a reinforcing member and the like for automobile doors.

CONSTITUTION: In a seam welded steel pipe in a round shape in cross section to be used as a member subjected to three point bending moment, a lightweight seam welded steel pipe excellent in three point bending characteristics is provided wherein the thickness (t1) of the pipe in a range where the maximum bending moment is incurred, is thickened over an appropriate length (S). As raw material, a seam welded steel pipe in a round shape in cross section is used, which can be manufactured with ease and also is economical. Since the steel pipe excellent in three point bending strength can be obtained by thickening a part of the steel pipe only by an appropriate length, the aforesaid steel pipe is advantageous in weight saving and is also economical.