Abstract: In order to provide a welding system capable of freely controlling dispersion and concentration of arc heat input at a groove face of base material and enable to ensure melting of the base material while restraining excessive weld heat input by carrying out a control of a heat input distribution of arc, in arc welding by a wire consumable electrode system, at a groove between welded members, a position of a weld torch or feed speed of a wire is periodically varied and welding is carried out by controlling a phase difference between the variation and an arc current characteristic when it is changed.