Provided is a battery capable of preventing the entry of water even if a sealing width is reduced. A battery element comprising a cathode and an anode is accommodated in a film-shaped casing. The casing includes a metal layer, a resin layer disposed on a side of the metal layer closer to the battery element with an adhesive layer in between, and a resin layer disposed on a side of the metal layer opposite to the side where the resin layer is formed with an adhesive layer in between. The adhesive layer has a water vapor transmission rate of 800 g/m.sup.2day or less for a thickness of 25 .mu.m at 40.degree. C. and 90% RH and a thickness of 10 .mu.m or less. Thereby, even if the sealing width is reduced, the entry of water into the battery can be prevented.