A flat secondary battery comprising: a power generation element including a positive electrode plate and a negative electrode plate stacked with a separator interposed between the plates; and an exterior body housing the power generation element and an electrolyte solution together and sealed by a laminated film having a heat-seal layer stacked on at least an inner surface of a metal layer, characterized in that a terminal (2, 3) having one end connected to the power generation element and the other end led out of the exterior body, held by the exterior body between the one end and the other end, and having a held portion, which is held by bonding surfaces of the laminated films of the exterior body, being sealed, the terminal (2, 3) includes a terminal main body including a nickel plane on a surface thereof, an anticorrosive layer covering at least a part of the nickel plane that is more on the inside of the exterior body than a held portion, and a resin layer covering at least the held portion of a surface of the anticorrosive layer and having an internal extension portion extending from the held portion to the inside of the exterior body, and the held portion is sealed by heat-sealing the resin layer and the heat-seal layer on the inner surface of the exterior body, wherein the internal extension portion extends from the held portion to the inside of the exterior body by 0.5 mm to 5 mm, wherein the exterior body is sealed under reduced pressure and a liquid volume coefficient of the electrolyte solution being a proportion of the electrolyte solution relative to a total pore volume of the positive electrode plate, the negative electrode plate and the separator is 1.1 to 1.6, wherein the power generation element and the terminal (2, 3) are bonded to each other through a connection portion interposed between the power generation element and the terminal (2, 3), and the thickness of bonded portion of the connection portion after the bonding is larger than the thickness of the internal extension portion. a power generation element including a positive electrode plate and a negative electrode plate stacked with a separator interposed between the plates; and an exterior body housing the power generation element and an electrolyte solution together and sealed by a laminated film having a heat-seal layer stacked on at least an inner surface of a metal layer, characterized in that a terminal (2, 3) having one end connected to the power generation element and the other end led out of the exterior body, held by the exterior body between the one end and the other end, and having a held portion, which is held by bonding surfaces of the laminated films of the exterior body, being sealed, the terminal (2, 3) includes a terminal main body including a nickel plane on a surface thereof, an anticorrosive layer covering at least a part of the nickel plane that is more on the inside of the exterior body than a held portion, and a resin layer covering at least the held portion of a surface of the anticorrosive layer and having an internal extension portion extending from the held portion to the inside of the exterior body, and the held portion is sealed by heat-sealing the resin layer and the heat-seal layer on the inner surface of the exterior body, wherein the internal extension portion extends from the held portion to the inside of the exterior body by 0.5 mm to 5 mm, wherein the exterior body is sealed under reduced pressure and a liquid volume coefficient of the electrolyte solution being a proportion of the electrolyte solution relative to a total pore volume of the positive electrode plate, the negative electrode plate and the separator is 1.1 to 1.6, wherein the power generation element and the terminal (2, 3) are bonded to each other through a connection portion interposed between the power generation element and the terminal (2, 3), and the thickness of bonded portion of the connection portion after the bonding is larger than the thickness of the internal extension portion.