A rectangular battery including a rectangular parallelepiped battery case made of metal and an electrode body enclosed in the battery case, wherein the battery case includes: a bottom-closed rectangular cylindrical main body member having a rectangular opening portion defined by a pair of opening long-side portions, a pair of opening short-side portions, and four opening rounded portions each curved to connect the opening long-side portions to the opening short-side portions; and a rectangular plate-like lid member inserted in the opening portion to close the opening portion, the lid member including a lid peripheral edge portion defined by a pair of lid long-side portions placed respectively to face the pair of opening long-side portions, a pair of lid short-side portions placed respectively to face the pair of opening short-side portions, and four lid rounded portions placed respectively to face the four opening rounded portions, wherein, in the opening portion of the main body member, the four opening rounded portions each include a support projecting portion projecting inward over the corresponding entire opening rounded portion in a circumferential direction of the opening portion to support the lid member inserted in the opening portion, characterized in that the pair of opening short-side portions each include a low projecting portion projecting inward over the entire opening short-side portion in the circumferential direction of the opening portion, each of the low projecting portions being positioned lower in a depth direction of the main body member than the support projecting portions, the four opening rounded portions and the four lid rounded portions have clearances one each between each opening rounded portion and the corresponding lid rounded portion, and are welded to each other by an energy beam irradiated to the lid member from outside in a thickness direction of the lid member, and the pair of opening short-side portions and the pair of lid short-side portions have a clearance at least either between the one opening short-side portion and the one lid short-side portion or between the other opening short-side portion and the other lid short-side portion, and are welded to each other by the energy beam. a bottom-closed rectangular cylindrical main body member having a rectangular opening portion defined by a pair of opening long-side portions, a pair of opening short-side portions, and four opening rounded portions each curved to connect the opening long-side portions to the opening short-side portions; and a rectangular plate-like lid member inserted in the opening portion to close the opening portion, the lid member including a lid peripheral edge portion defined by a pair of lid long-side portions placed respectively to face the pair of opening long-side portions, a pair of lid short-side portions placed respectively to face the pair of opening short-side portions, and four lid rounded portions placed respectively to face the four opening rounded portions, wherein, in the opening portion of the main body member, the four opening rounded portions each include a support projecting portion projecting inward over the corresponding entire opening rounded portion in a circumferential direction of the opening portion to support the lid member inserted in the opening portion, characterized in that the pair of opening short-side portions each include a low projecting portion projecting inward over the entire opening short-side portion in the circumferential direction of the opening portion, each of the low projecting portions being positioned lower in a depth direction of the main body member than the support projecting portions, the four opening rounded portions and the four lid rounded portions have clearances one each between each opening rounded portion and the corresponding lid rounded portion, and are welded to each other by an energy beam irradiated to the lid member from outside in a thickness direction of the lid member, and the pair of opening short-side portions and the pair of lid short-side portions have a clearance at least either between the one opening short-side portion and the one lid short-side portion or between the other opening short-side portion and the other lid short-side portion, and are welded to each other by the energy beam.