As the vinylidene chloride copolymer, there is used a copolymer comprising vinylidene chloride as the main constituent monomer and at least one comonomer selected from an acrylic or methacrylic monomer, a vinyl aromatic monomer such as styrene or vinyl toluene, a vinyl ester such as vinyl acetate or vinyl propionate, a diolefin such as butadiene or isoprene, and methyl vinyl ether, glycidyl allyl