Examples of the material of this heat-sealing layer are resins such as polyethylene, polypropylene, an ethylene-vinyl acetate copolymer, an ethylene-methacrylic acid copolymer, an ethylene-methacrylic ester copolymer, an ethylene-acrylic acid copolymer, an ethylene-acrylic ester copolymer, and metal crosslinked products of these materials.