A plurality of battery cells are connected by a busbar. Electrode terminals of the battery cells each include a protruding portion and a welding surface around the protruding portion. The busbar includes a welding plate portion being in surface-contact with the welding surface and having a cut-away portion for guiding the protruding portion, and an exposure gap that exposes the welding surface between the inner side of the cut-away portion and the protruding portion. The busbar is welded to the welding surface in a predetermined welding width (H) by both of a fillet weld part and a penetration welding portion such that the inner edge of the cut-away portion as the fillet weld part is welded to the welding surface, and the boundary between the busbar and the welding surface is welded by the penetration welding portion.