A flat battery, comprising: a bottomed cylindrical exterior can having a cylindrical side wall extending in a cylinder axial direction; a bottomed cylindrical seal can having a peripheral wall extending in the cylinder axial direction and covering an opening of the exterior can such that the peripheral wall is located inside the exterior can; and a gasket formed by covering the peripheral wall of the seal can and sandwiched between the exterior can and the seal can, wherein the gasket includes: a covering portion molded on the peripheral wall so as to cover at least a portion of the peripheral wall of the seal can, the covering portion also covering an inner side of the seal can; and a protruding portion formed integrally with the covering portion and displaced from the peripheral wall in the cylinder axial direction, wherein: a portion of the covering portion that is located adjacent the exterior can is sandwiched between the cylindrical side wall of the exterior can and the peripheral wall of the seal can, an injection portion is located on a portion of the protruding portion that faces the cylindrical side wall of the exterior can, and the injection portion has a recess formed on a surface of the protruding portion. a bottomed cylindrical exterior can having a cylindrical side wall extending in a cylinder axial direction; a bottomed cylindrical seal can having a peripheral wall extending in the cylinder axial direction and covering an opening of the exterior can such that the peripheral wall is located inside the exterior can; and a gasket formed by covering the peripheral wall of the seal can and sandwiched between the exterior can and the seal can, wherein the gasket includes: a covering portion molded on the peripheral wall so as to cover at least a portion of the peripheral wall of the seal can, the covering portion also covering an inner side of the seal can; and a protruding portion formed integrally with the covering portion and displaced from the peripheral wall in the cylinder axial direction, wherein: a portion of the covering portion that is located adjacent the exterior can is sandwiched between the cylindrical side wall of the exterior can and the peripheral wall of the seal can, an injection portion is located on a portion of the protruding portion that faces the cylindrical side wall of the exterior can, and the injection portion has a recess formed on a surface of the protruding portion. a covering portion molded on the peripheral wall so as to cover at least a portion of the peripheral wall of the seal can, the covering portion also covering an inner side of the seal can; and a protruding portion formed integrally with the covering portion and displaced from the peripheral wall in the cylinder axial direction, wherein: a portion of the covering portion that is located adjacent the exterior can is sandwiched between the cylindrical side wall of the exterior can and the peripheral wall of the seal can, an injection portion is located on a portion of the protruding portion that faces the cylindrical side wall of the exterior can, and the injection portion has a recess formed on a surface of the protruding portion. a portion of the covering portion that is located adjacent the exterior can is sandwiched between the cylindrical side wall of the exterior can and the peripheral wall of the seal can, an injection portion is located on a portion of the protruding portion that faces the cylindrical side wall of the exterior can, and the injection portion has a recess formed on a surface of the protruding portion.