A flat secondary battery has a laminate-type power generation element in which two or more plate-like electrodes are laminated via 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 that seal the power generation element and an electrolyte solution. 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.