FLAT SECONDARY BATTERY

A flat secondary battery characterized by comprising: a laminate-type power generation element in which two or more plate-like electrodes are laminated via each of separators; and a pair of rectangular exterior members defined by long sides and short sides when viewed from a lamination direction of the two or more electrodes, the rectangular exterior members sealing the laminate-type power generation element and an electrolyte solution, wherein at least one exterior member of the pair of the rectangular exterior members comprises: an abutting part including an abutting surface that abuts against an uppermost layer electrode of the two or more electrodes; a sealing part at which the rectangular exterior members overlap each other at an outer circumferential position of the rectangular exterior members; and an extending part that extends from the abutting part to the sealing part, and the flat secondary battery satisfies 1¤LA/LB¤2. Here the L A represents an average of lengths of a pair of the extending parts located at both ends in a direction parallel to the long sides in a first cross section and the first cross section is a cross section of the flat secondary battery when cutting through the flat secondary battery by a plane parallel to the long sides and along the lamination direction, and the L B represents an average of lengths of a pair of the extending parts located at both ends in a direction parallel to the short sides in a second cross section and the second cross section is a cross section of the flat secondary battery when cutting through the flat secondary battery by a plane parallel to the short sides and along the lamination direction.