PURPOSE: To eliminate center displacement of a disc-like negative electrode, and to provide a flat battery having stable discharging characteristic, in which a power generation factor comprising the disc-like negative electrode, a separator, a positive electrode molded into pellet, and an electrolyte, is sealed with a sealing plate, a metal case, and a gasket sandwiched between them.

CONSTITUTION: The cross section of a gasket 13 which is a member forming a flat battery is formed into recessed shape, and a tapered starting part 13a of the inner circumference of the recessed part form a thin part 13b which is extended in the inner and upper direction. The dimensional relation between a minimum inner diameter L2 of the thin part 13b of the gasket 13 and a diameter L1 of a disc-shaped negative electrode 15 is set to L1-L2≥0. Center displacement at the time of inserting the disc-shaped negative electrode 15 into a sealing plate 11 is prevented effectively by means of this two-point effect.