A gas-barrier resin (A) having an oxygen permeability coefficient of 1.0×10−14 (cm3·cm/cm2·s·Pa) or less; and a copolymer (B) containing monomer structural units represented by the formula (1), the formula (2), and the formula (3):




where: R1 represents a hydrogen atom or a methyl group; R2 represents a hydrocarbon group having 1 to 20 carbon atoms that may be substituted with a halogen atom, a hydroxy group, an alkoxy group, or an amino group; 1, m, and n represent numerical values representing molar proportions of the respective monomer structural units, and n may represent 0; and p represents an integer of from 1 to 4, wherein a ratio of a mass of the copolymer (B) to a total mass of the gas-barrier resin (A) and the copolymer (B) is from 1 mass % to 40 mass %.