The present invention pertains to a welding device and a welding method performed via an algorithm that controls a torch aim location and via welding conditions that do not cause welding defects for fluctuations during welding in ultra-narrow gap SAW. The bead width WB and deposit cross-sectional area AR when bead-on-plate welding has been performed on a material that is the same as that of a welding target are measured. The heat input HG to a gap wall during welding is represented using a welding heat input Q, the bead width WB, and a distance L between torch gap walls. A generalized distance R from a weld pool surface at the torch aim location to a corner of a gap bottom is represented using the distance L and the deposit cross-sectional area AR. On an HG-R plane, a display is plotted for the experimentally-obtained presence/absence of fusion flaws occurring during gap welding, and an assessment line based on the presence/absence of fusion flaws in the gap walls is drawn. The gap welding is performed by setting the welding conditions so as to form a region that has less fusion flaws than this assessment line.