A flat battery comprising: an exterior can shaped as a cylinder with a bottom; a seal can having a cylinder portion with an outer shape that is smaller than a sidewall of the exterior can and a flat portion that closes up one opening of the cylinder portion, the seal can being disposed as an inverted dish with respect to the exterior can to form a space between itself and the exterior can; and a gasket molded on an inner surface of the seal can, extending from an opening edge of the cylinder portion to the flat portion, wherein the cylinder portion of the seal can has an outspread portion, defined by a step, with a larger diameter than a base edge portion which is close to the flat portion and a stepped portion at which an opening edge portion of the cylinder portion is expanded from the step formed between the base edge portion and the outspread portion, an opening edge of the sidewall of the exterior can is fitted onto the stepped portion of the seal can, the gasket includes an inner gasket portion covering the inner side of the cylinder portion with respect to the seal can, the inner gasket portion includes an upper gasket portion located inside the base edge portion with respect to the seal can and a lower gasket portion (31 b) located inside the stepped portion and outspread portion with respect to the seal can, the gasket is molded on an outer side of the stepped portion and the opening edge portion of the cylinder portion, a portion of the lower gasket portion (31 b) that is close to the upper gasket portion (31 a) has an inner tapered portion (31 c) such that its inner diameter gradually increases from the upper gasket portion toward the lower gasket portion so that the portion of the lower gasket portion (31 b) of the gasket that is located on the inside of the opening edge portion of the cylinder portion has a thickness smaller than the thickness of the portion of the lower gasket portion (31 b) of the gasket that is located on the inside of the stepped portion, and the gasket further comprises a gasket edge portion sandwiched between the opening edge of the cylinder portion and the bottom of the exterior can. an exterior can shaped as a cylinder with a bottom; a seal can having a cylinder portion with an outer shape that is smaller than a sidewall of the exterior can and a flat portion that closes up one opening of the cylinder portion, the seal can being disposed as an inverted dish with respect to the exterior can to form a space between itself and the exterior can; and a gasket molded on an inner surface of the seal can, extending from an opening edge of the cylinder portion to the flat portion, wherein the cylinder portion of the seal can has an outspread portion, defined by a step, with a larger diameter than a base edge portion which is close to the flat portion and a stepped portion at which an opening edge portion of the cylinder portion is expanded from the step formed between the base edge portion and the outspread portion, an opening edge of the sidewall of the exterior can is fitted onto the stepped portion of the seal can, the gasket includes an inner gasket portion covering the inner side of the cylinder portion with respect to the seal can, the inner gasket portion includes an upper gasket portion located inside the base edge portion with respect to the seal can and a lower gasket portion (31 b) located inside the stepped portion and outspread portion with respect to the seal can, the gasket is molded on an outer side of the stepped portion and the opening edge portion of the cylinder portion, a portion of the lower gasket portion (31 b) that is close to the upper gasket portion (31 a) has an inner tapered portion (31 c) such that its inner diameter gradually increases from the upper gasket portion toward the lower gasket portion so that the portion of the lower gasket portion (31 b) of the gasket that is located on the inside of the opening edge portion of the cylinder portion has a thickness smaller than the thickness of the portion of the lower gasket portion (31 b) of the gasket that is located on the inside of the stepped portion, and the gasket further comprises a gasket edge portion sandwiched between the opening edge of the cylinder portion and the bottom of the exterior can.