A lithium battery (1) has a battery module (2), a battery package (5) for holding the battery module (2) therein, and leads (4) extending from the battery module (2) and projecting outside from the battery package (5). The battery package (5) is formed from a laminated sheet (10), and a peripheral part (9) of the battery package (5) is heat-sealed. Lead-insulating films (6) are interposed between the peripheral part (9) of the package (5) and the leads (4). Each of the lead-insulating films includes a heat-resistant base film (22), a polyolefin resin layer (21) formed on one of the surfaces of the base film (22) on the side of the laminated sheet (10), and an acid-modified polyolefin resin layer (23) formed on the other surface of the base film (22) on the side of the lead (4).