Film covered battery and stacked battery assembly

A film covered battery has a battery element, a positive and a negative lead terminal connected to the battery element, and a casing for sealing the lead terminals and the battery element together with an electrolytic solution, with portions of the lead terminals extending therefrom. The casing has a buffer in at least a portion thereof for accumulating a gas generated within the casing through deformation of the casing. The buffer is formed continuously with a region for accommodating the battery element.