Provided is a flat battery with a molded gasket extending from the opening edge of the seal can to the flat portion, where the gasket may be prevented from exfoliating from the inner surface of the seal can. A flat battery (1) includes: a negative electrode can (10) (exterior can) shaped as a cylinder with a bottom; a positive electrode can (20) (seal can) having a peripheral wall (22) (cylinder portion) and a flat portion (21), the positive electrode can being disposed as an inverted dish with respect to the negative electrode can (10); and a gasket (30) molded at least on an inner surface of the positive electrode can (20), extending from the opening edge of the peripheral wall (22) to the flat portion (21). The opening edge of the sidewall of the negative electrode can (10) is fitted onto the stepped portion (22c) of the positive electrode can (20), and a portion of the gasket (30) that is located inside the opening edge portion of the peripheral wall (22) of the positive electrode can (20) with respect to the positive electrode can has a thickness smaller than that of a portion of the gasket that is located inside the stepped portion (22c) of the positive electrode can (20) with respect to the positive electrode can.