To suppress a curling by a method wherein a sheet consisting of a blend of polypropylene and a melting mixture of a specified ratio based on the total amount of a composition is thermally formed, and an aluminum foil is heat-sealed using an adhesive within a specified temperature range.

The blending amount of polypropylene is 85-50wt.% based on the total amount of the composition. The refraction factor difference with polypropylene at a normal temperature, is 0.025 or lower, and a viscosity ratio between the melt viscosity which is measured by a capillograph IC at 250°C, by a shear velocity speed of 121.6sec-1, and the melt viscosity of polypropylene, is 0.05-1.0. The blending amount of a molten mixture of polyethylene or an ethylene -α- olefin copolymer is 15-50wt.%. In order to suppress a curling, a heat-sealing is performed at a low temperature. Also, a bonding between aluminum foil and the sheet is made possible without melting polypropylene.