A secondary battery comprises a flat wound electrode body, a flat-type hard case housing the wound electrode body, and a film sandwiched between them. The film is formed with thick portions each of which is located at a boundary region between a flat portion and a curved portion of the surface of the wound electrode body. When a plurality of the secondary batteries are bound, therefore, load is applied widely on almost the entire wound electrode body, thereby enabling exertion of sufficient generation capability. Thus, a secondary battery with improved battery performance is provided in which load is sufficiently applied on an electrode body. The secondary battery can also be used as a battery assembly, a battery to be mounted on a vehicle, and a battery to be mounted on a battery mounting device.