To safely, quickly replace a valve at the front end of a pipeline provided on a storage tank for a liquefied gas or the like while the tank is practically used.

A sealed chamber 3 is formed of a cylindrical body 3d with the same diameter as that of a lateral 9 and having a flange part 3a freely connected to the opened side of a valve 11 at one end and sealed by an end plate 3c having a through hole 3b for penetrating an inert gas supply pipe body 5 at the other end. The inert gas supply pipe body 5 is formed of an operating pipe part 5a slidably interposed in the through hole 3b provided in the end plate 3c of the sealed chamber 3, a sealing pipe part 5b for mounting a bag body 7 for sealing the inner part of the lateral 9 at its intermediate part, and a bent pipe part 5c for connecting the operating pipe part 5a to the sealing pipe part 5b, and for shifting the position of the operating pipe part 5a at its central axis line from the position of the sealing pipe part 5b at its central axis line, and for retaining the axis lines in parallel.