To provide a manufacturing method by which a double pipe having an outer pipe part which has high rigidity is manufactured even when a thin tubular material is used.

After forming a collar part 12 by expanding the lower end part of a tubular material 1, the lower end face of the collar part 12 is brought into contact with the contacting surface 21 of the collar part of a die 2 and a shaft material 4 is inserted into the main body part 13 by using the die 2 having the contacting surface 21 of the collar part with which the lower end part of the collar part 12 can be contacted and a die hole 24 which is formed into a shape in which at least a part of the inner peripheral surface 24c is expanded as it recedes from the contacting surface 21 of the collar part. By applying the pressure of liquid to the root part 12a of the collar part 12 connecting to the main body part 13 from the upper part in this state, the main body part 13 is turned so as to reverse the outer peripheral surface and the inner peripheral surface while expanding the main body part 13 successively from the end part and also the outer pipe part 14 having a shape matching the inner peripheral surface 24a of the die hole 24 is formed by pressing the turned part against the inner peripheral surface 24a of the die hole 24.

COPYRIGHT: (C)2006,JPO&NCIPI