PURPOSE: To make the welded ends of both pipe materials thicker than the other parts even after welding and to improve safety by forming said ends thicker than the other parts, machining the inside circumferential surfaces of the ends and subjecting the ends of both pipe materials to butt welding.

CONSTITUTION: Both ends of pipe materials 1, 1 are formed thicker than the other parts of the materials 1, 1. The inside circumferential parts of the ends are machined 2 to match the bores thereof and thereafter the ends of the materials 1 are beveled 3 and a pair of the materials 1, 1 are butt-welded via a weld zone 4 to manufacture a pipe structural body. The wall thickness (c) of the weld zone 4 is thickner than the wall thickness (a) of the intermediate part even after the machining 2 and therefore the stress in the zone 4 is decreased and the strength of the weld zone 4 is increased by which the pipe structural body having high strength is obtd. Since the ends are machined 2, no adverse influence is given to the non-destructive inspection after welding.