PURPOSE: To provide a method for bending a tube with irregular shaped section, by which the tube with irregular shaped section can be normally bent without being deformed to a flat, and to provide the tube with irregular shaped section, which can be suitably used for this bending method.

CONSTITUTION: In a method for bending a tube with irregular shaped section, an outer core 4 is formed by repeating deformed part 2 and nondeformed part 3 alternately in the peripheral direction. It is the method for bending a tube 1 with irregular shaped section, which is made possible to contract a diameter by deforming the deformed part 2 in a direction to which adjacent nondeformed parts 3 approach each other by contracting the diameter. Bending is performed at a state in which a reinforcing material 5 for preventing a deformation is allowed to exist, which blocks that the tube becomes smaller than a specific tube diameter, inside a prearranged position where the tube 1 is bent. Also, in the tube with irregular shaped section, a rib 5 blocking that the tube becomes smaller than a specific tube diameter by abutting each other in the minimum diameter contraction state, is provided projectingly as one body on the inside surface of each nondeformed part 3 of the outer core 4 toward the center of the outer core 4.