FLAT-TYPE SECONDARY BATTERY

A flat secondary battery 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 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 1.03 ≤ L b 2 + d 2 ≤ 1.22 wherein, in a cross section of the flat secondary battery when cutting through the flat secondary battery by a plane parallel to sides that form the rectangular exterior members and along the lamination direction, the L represents a length of the extending part, the b represents a length from an inter-sealing part electrode of the two or more electrodes to the sealing parts, the inter-sealing part electrode is located at height of the sealing part in the lamination direction, and the d represents a thickness of the laminate-type power generation element from the inter-sealing part electrode to the uppermost layer electrode.