Saponified and its manufacturing method of a vinyl ester / ethylenically unsaturated carboxylic acid ester copolymer

Providing a saponified vinyl ester / ethylenically unsaturated carboxylic acid ester copolymer having excellent resistance to mechanical shearing of the aqueous increase mucus. More specifically, by polymerizing a mixture molar ratio of vinyl ester / ethylenically unsaturated carboxylic acid ester is 8 / 2-3 / 7 in the monomer, the resulting copolymer in the aqueous organic solvent / water mass composition ratio by saponifying with 3 / 7-8 / 2 in which a mixed solvent provides a saponified vinyl ester / ethylenically unsaturated carboxylic acid ester copolymer having a superior resistance.