To reduce time required for assembly by making it possible to continu ously weld flanges of sheaths.

To weld together flanges 19 of sheaths for a SF6 gas insulated cable, the method uses an openable clamping device 29 that has a large number of clamps 37, 39 arranged at appropriate intervals in the circumferential direction and a opening/closing mechanism 43 for individually opening and closing the clamps. The butt-joined sections of the flanges 19 are fastened together using the openable clamping device 29, and are weld together by relatively shifting a welding torch 27 in the circumferential direction. When the welding torch 27 passes by during this operation, clamps 39 are sequentially opened to allow continuous welding.