PURPOSE: To obtain a bent pipe having the uniform wall thickness throughout its whole body by bending by making the wall thickness at the bent inside of a predetermined part to be bent of an original pipe thinner than that of a straight part and making the wall thickness at the bent outside thicker than that of the straight part, and forming the original pipe into a blank pipe for bending by making its outer surface flush with each other.

CONSTITUTION: An original pipe 1 is cut so as to form the cross section of the part 1a to be the bent inside of a part 11 predetermined to be bent, into a crescent through an approximately semicicumference of pipe 1. Successively, a predetermined part 12 to be used as a straight part is drawn by using a die 5 and a plug 6 to form the pipe 1 into the shape of a prescribed blank pipe 10; then the blank pipe 10 for a bent pipe is formed by making the wall thickness of the bent inside 1a thin and that of the bent outside thick and making the outer circumferential surfaces of the parts 11 and 12 flush with each other.