To provide a method and a device for attaching an elongated/contracted flexible joint, wherein a work process is simple, and manufacturing costs are lowered.

An elongated/contracted flexible joint 5 is temporarily installed in a fluid pipe 6, a case body 1 is attached to cover the elongated/contracted flexible joint 5, the elongated/contracted flexible joint 5 is moved to a position for covering a cut part after a cutter is inserted from an opening part provided in the case body 1 to cut the fluid pipe 6, and by removing the case body 1, the elongated/contracted flexible pipe 5 is attached. Thus, since the elongated/contracted flexible pipe 5 is attached to the fluid pipe 6 without using any working gate valves, the number of work steps is reduced and the number of times of using a heavy machine is reduced, costs are lowered. In addition, since a boring machine is provided in the opening part of the case body 1 to cut the fluid pipe 6 by the cutter, the conventional necessity of attaching the boring machine and providing a main cylindrical part for the insertion/ pulling-out of the cutter is eliminated for the elongated/contracted flexible joint 5, the manufacturing costs of the elongated/contracted flexible joint 5 are reduced.