NOVELTY - A battery pack comprises battery element (59); generator device having the battery element disposed on a flexible covering material being folded to cover the battery element, with 3 sides of the flexible covering material around the battery element are sealed; rigid covering material for covering the generator device, having an opening and being bonded to the flexible covering material of the generator device; and cover fitted into the opening of the rigid covering material.    USE - For lithium ion secondary battery.    ADVANTAGE - The battery pack is thin and has excellent volume efficiency, predetermined battery strength, excellent sealing properties, and excellent heat radiation properties.    DETAILED DESCRIPTION - The battery pack comprises battery element; generator device having the battery element disposed on a flexible covering material being folded to cover the battery element, with 3 sides of the flexible covering material around the battery element are sealed; rigid covering material for covering the generator device, having an opening and being bonded to the flexible covering material of the generator device; and cover fitted into the opening of the rigid covering material. The rigid covering material has a 4-layer structure having an outer packaging layer, a metal layer, an inner packaging layer, and a heat-bonding layer which are stacked on one another. The heat-bonding layer is a material consisting of ethylene-vinyl acetate copolymer, ethylene-acrylic acid copolymer, ethyl acrylate copolymer, methyl acrylate copolymer, methacrylic acid copolymer, methyl methacrylate copolymer, polyacrylonitrile, ethylene vinyl alcohol resin, polyamide resin, polyester resin, acid-modified polypropylene, or ionomer.    DESCRIPTION OF DRAWING(S) - The figure is a diagrammatic view of the battery element.    Positive electrode (51)    Negative electrode (52)    Anode active material (52a)    Negative anode current collector (52b)    Separator (53b)    Battery element (59)