To provide a method and device for securely bending a double tube efficiently and economically with a simple structure.

One end 10A of a tube body 10 to be bent is sealed with a lid member 30, and a plug member 40 is mounted on the other end 10B. The plug member 40 gas a small diameter part 41 set in the inner wall 12B of an inner tube and a large diameter part 45 with an inner part 46 pressed on the end edge 12C of the inner tube, and also has communicating parts 47 and 48 communicating form the large diameter part with the inside 12D of the inner tube and the gap part. A coupler member 50 is mounted on the outside of the plug member. The coupler member 50 is pressed on the outer surface 11b of the outer tube to form a storage space to store a pressure liquid F between the coupler member and the large diameter part of the plug member, and the storage space, the inside of the inner tube and the gap part are filled with the pressure liquid to seal and bend the double tube.