A battery including: a case body having a flattened shape and including an open end and internally containing a power generating element; and a lid member formed in a shape having long side parts and short side parts and placed to close an opening of the case body, wherein the battery further includes: external terminal members (130,140) provided to penetrate through the lid member and connected to the power generating element in the case body and partially exposed to outside; and insulating members placed on at least an upper surface of the lid member to insulate the external terminal member from the lid member, an interval between an edge of the insulating member and a long side of the lid member is smaller than an interval between an edge of the insulating member and a short side of the lid member, the lid member is fitted in the opening of the case body, a weld mark extending across the upper surface of the lid member and an open end face of the case body is formed along an entire circumference of the lid member so that the lid member is fixed to the case body and inside of the case body is hermetically sealed from external environment, a zone (Z1, Z3) of the lid member along the long side at least facing the insulating member is a wide-width weld mark zone in which the weld mark is formed to reach an outer side surface of the case body in addition to the upper surface of the lid member and the open end face of the case body, and a zone of the lid member along the short side and along the long side except the zone (Z1, Z3) is a zone in which the weld mark has a smaller width than that of the wide-width weld mark zone, wherein the weld mark is formed to have a weld spreading depth (Ma) of a cross section that is larger in the wide-width weld mark zone than in a zone except the wide-width weld mark zone, and the weld mark is formed to be spread into the lid member from an extended line extended to a surface of the weld mark from a boundary between the lid member and the case body in an unmelted portion during welding, and wherein the external terminal member and the insulating member are respectively provided in positions close to both ends of the lid member in a long side direction. external terminal members (130,140) provided to penetrate through the lid member and connected to the power generating element in the case body and partially exposed to outside; and insulating members placed on at least an upper surface of the lid member to insulate the external terminal member from the lid member, an interval between an edge of the insulating member and a long side of the lid member is smaller than an interval between an edge of the insulating member and a short side of the lid member, the lid member is fitted in the opening of the case body, a weld mark extending across the upper surface of the lid member and an open end face of the case body is formed along an entire circumference of the lid member so that the lid member is fixed to the case body and inside of the case body is hermetically sealed from external environment, a zone (Z1, Z3) of the lid member along the long side at least facing the insulating member is a wide-width weld mark zone in which the weld mark is formed to reach an outer side surface of the case body in addition to the upper surface of the lid member and the open end face of the case body, and a zone of the lid member along the short side and along the long side except the zone (Z1, Z3) is a zone in which the weld mark has a smaller width than that of the wide-width weld mark zone, wherein the weld mark is formed to have a weld spreading depth (Ma) of a cross section that is larger in the wide-width weld mark zone than in a zone except the wide-width weld mark zone, and the weld mark is formed to be spread into the lid member from an extended line extended to a surface of the weld mark from a boundary between the lid member and the case body in an unmelted portion during welding, and wherein the external terminal member and the insulating member are respectively provided in positions close to both ends of the lid member in a long side direction.