A method for welding electrode tabs protruding from a plurality of electrode plates, comprising: a process of forming a tab arrangement by arranging the electrode tabs vertically so that at least parts of the electrode tabs overlap each other; a process of irradiating light to at least two side surfaces among side surfaces of the tab arrangement excluding surfaces on which the electrode tabs overlap and surfaces facing the overlapped surfaces; a process of scanning a planar shape of a step on which the electrode tabs do not overlap each other using light reflected from an end portion of each of the electrode tabs; a process of scanning only an overlapped portion of the electrode tabs excluding the step based on the scanned result; a process of setting a welding range within the overlapped portion; and a process of performing welding only within the welding range, wherein the process of scanning the shape of the step comprises: (i) a process of recognizing a first boundary shape between the electrode tabs on a first side surface while light which is incident on the first side surface is being reflected at the end portion of each of the electrode tabs on the first side surface; and (ii) a process of recognizing n (n being one or more) boundary shapes between the electrode tabs on n side surfaces while light which is incident on the n side surfaces, which are located in directions different from the first side surface, is being reflected at the end portion of the electrode tab located on each of the n side surfaces, and based on any one of the boundary shapes, the remaining boundary shapes are combined in a planar manner so that a planar shape formed by the steps is scanned, and the overlapped portion of the electrode tabs are further scanned. a process of forming a tab arrangement by arranging the electrode tabs vertically so that at least parts of the electrode tabs overlap each other; a process of irradiating light to at least two side surfaces among side surfaces of the tab arrangement excluding surfaces on which the electrode tabs overlap and surfaces facing the overlapped surfaces; a process of scanning a planar shape of a step on which the electrode tabs do not overlap each other using light reflected from an end portion of each of the electrode tabs; a process of scanning only an overlapped portion of the electrode tabs excluding the step based on the scanned result; a process of setting a welding range within the overlapped portion; and a process of performing welding only within the welding range, wherein the process of scanning the shape of the step comprises: (i) a process of recognizing a first boundary shape between the electrode tabs on a first side surface while light which is incident on the first side surface is being reflected at the end portion of each of the electrode tabs on the first side surface; and (ii) a process of recognizing n (n being one or more) boundary shapes between the electrode tabs on n side surfaces while light which is incident on the n side surfaces, which are located in directions different from the first side surface, is being reflected at the end portion of the electrode tab located on each of the n side surfaces, and (i) a process of recognizing a first boundary shape between the electrode tabs on a first side surface while light which is incident on the first side surface is being reflected at the end portion of each of the electrode tabs on the first side surface; and (ii) a process of recognizing n (n being one or more) boundary shapes between the electrode tabs on n side surfaces while light which is incident on the n side surfaces, which are located in directions different from the first side surface, is being reflected at the end portion of the electrode tab located on each of the n side surfaces, and based on any one of the boundary shapes, the remaining boundary shapes are combined in a planar manner so that a planar shape formed by the steps is scanned, and the overlapped portion of the electrode tabs are further scanned.