A battery body, which is composed of a laminated body obtained by laminating a cathode and an anode via a separator or a wound body obtained by winding them in a laminated structure and a plurality of leads connected to the cathode and the anode, is contained in a flexible package, and after an electrolyte solution is injected thereinto, an opening of the package is sealed at pressure which is lower than atmospheric pressure and is reduced to not less than vapor pressure of the electrolyte solution so that a thin battery is fabricated. A rise in pressure in the battery due to generation of gas in the battery and volume expansion of the gas can be restrained when the battery is maintained at high temperature.