A welding system for superhigh purity gas supply pipe system, capable of removing easily and completely the metals deposited, during the installation of the superhigh purity gas pipe system, on the outer surfaces of the portions of the pipes which are closed to the portions to be welded thereof and on the inner portions of these pipes, and capable of setting up a superhigh purity gas supply system in a short period of time. A welding system for the gas supply pipe system in which a plurality of members are connected together by welding, characterized in that a first member to be welded (113) is provided with a means for supplying an inert gas or a welding back seal gas and a means for supplying superpure water, a second member to be welded (114) being provided with a means for discharging the inert gas or back seal gas and a means for discharging the superpure water, the first and second members being welded as the inert gas or back seal gas is being supplied, the superpure water being supplied after the welding of these members has been completed, whereby the metal fume deposited on the surfaces of the welded members during the welding thereof can be washed out.