To efficiently and inexpensively obtain a double tube of excellent heat exchange performance having a bent part to which accurate bending is performed while allowing a space between an outer tube and an inner tube of the bent part to be made a desired space without filling the tube with a filler for preventing the crush of the tube.

The inner tube provided with at least one spiral worked part of desired length in a tube axis direction having a spiral outer diameter slightly smaller than the inner diameter of the outer tube is inserted in the outer tube, and the outer tube and the spiral worked part of the inner tube inserted in the outer tube are bent together at least at one part.