To provide a coupling method for a double-layered tube capable of coupling end parts of the double-layered tubes by assembling a plurality of end parts without using a coupling member and without changing cross sectional shapes of inner tubes, maintaining the interval of the inner tubes and outer tubes constant at the end parts of the double-layered tubes.

Plane parts 4 to be joined by being brought into face contact with each other are formed on side surfaces of the outer tubes 3 without changing the shapes of the inner tubes 2 at end parts of the double-layered tubes 1 constituted of the inner tubes 2 and the outer tubes 3 provided in constant space provided states at the outside. The end part of each of the double-layered tubes 1 is assembled so that the plane parts 4 formed on the side surfaces of the outer tubes 3 may be turned inside and each of the plane parts 4 is joined by being brought into face contact with each other.