NOVELTY - The battery (1) has a horizontal interface boundary (Fx) that is formed between a battery can (100) and a battery cover (200) and in a horizontal direction intersecting a height direction of a side wall portion. A vertical interface boundary (Fy) is formed in a vertical direction along the height direction of side wall portion. A structure is provided such that the portions of the horizontal and vertical interface boundaries are welded by a laser (EB) irradiated to the vertical direction along the height direction of the side wall portion.    USE - Square battery such as lithium ion battery mounted in vehicle such as electric vehicle (EV) and hybrid vehicle.    ADVANTAGE - The portions of the horizontal and vertical interface boundaries are welded by a laser irradiated to the vertical direction along the height direction of the side wall portion so that the force of the direction which peels the battery cover is resisted, and high welding strength is obtained. Thus, the stable welding strength is obtained effectively while suppressing the fluctuation in welding strength. Hence, the preset welding strength is secured easily. The battery cover makes low protrusion height of the downward convex portion by providing the upper convex portion in the upper surface and the interior volume of battery container is ensured larger when the opening portion of the battery can is obstructed, and only the portion to which the volume is large enlarges the size of flat winding group, and to attain high capacitation of battery. Thus, the fall of rigidity of battery cover is prevented effectively while improving the strength of the battery cover. The projection portion is melted by the laser, and is allotted to the welding replenishment metal of the welding portion so that the projection portion is melted in the portion, volume is large, and the stress on which a cooling rate acts by slowing at the time of the solidification shrinkage after welding is relieved. Thus, the crack of the welding portion is prevented effectively while obtaining high welding quality.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of the junctional portion of the battery can and battery cover.    Square battery (1)    Battery can (100)    Battery cover (200)    Laser (EB)    Horizontal interface boundary (Fx)    Vertical interface boundary (Fy)