A battery pack is provided. The battery pack includes: a generator device having a battery element disposed on a flexible covering material folded to cover the battery element, wherein three sides of the flexible covering material around the battery element are sealed; a rigid covering material for covering the generator device, having an opening and being bonded to the flexible covering material of the generator device; and a cover fitted into the opening of the rigid covering material, wherein the rigid covering material has a three-layer structure having an outer packaging layer, a metal layer, and a heat-bonding layer which are stacked on one another, and the heat-bonding layer is one selected from an ethylene-vinyl acetate copolymer, an ethylene-acrylic acid copolymer, an ethyl acrylate copolymer, a methyl acrylate copolymer, a methacrylic acid copolymer, a methyl methacrylate copolymer, polyacrylonitrile, an ethylene vinyl alcohol resin, a polyamide resin, a polyester resin, acid-modified polypropylene, or an ionomer.