To excellently bend a double tube of small diameter while an inner tube holder to regulate the bending displacement of the inner tube is dispensed with.

A local part of an inner circumference of an inner tube 1 is heated in the circumferential direction by a heating coil 6 for the inner tube, and the compressive load in the axial direction is given to the inner tube 1, and a heated part is locally swollen to form an annular inner tube projected part 1b in which a forward end outer circumferential surface is abutted on the inner circumferential surface of an outer tube 2. The outer circumferential surface of the outer tube 2 is restricted by an outer tube holder 9 at the position corresponding to the inner tube projected part 1b in the axial direction. An inner circumferential local part of the inner tube 1 and an outer-circumferential local part of the outer tube 2 are respectively heated by the heating coil 6 for the inner tube and a heating coil 7 for the outer tube in the circumferential direction at the position away from the inner tube projected part 1b and the outer tube holder 9 by the prescribed distance in the axial direction, and the inner tube 1 and the outer tube 2 are bent by applying the bending moment to the heated parts of the inner tube 1 and the outer tube 2 while the inner tube 1 and the outer tube 2 relatively moved in the axial direction to the outer tube holder 9.