To obtain a new polyvinyl-alcohol-based polymer which can form a gel excellent in flexibility, transparency, wet-heat resistance, hot-water resistance, water absorbency and occlusiveness by modifying a polyvinyl alcohol with a specified ertherifiable derivative.

A polyvinyl alcohol produced by a usual method is modified with a compound represented by the formula: Z-X (wherein Z is hydroxyl or a halogen: X is a group represented by the formula: -COY, -Y or -CO-COOH or forms a phospho group together with the O atom to which it is bonded; and Y is a hydrocarbon group modified with a group having at least one polar group selected among carboxyl, sulfo, amino and phosphate), an esterifiable derivative thereof and/or an etherifiable derivative thereof to obtain a polyvinyl alcohol-based-polymer containing structural units each represented by formula I (wherein R1 is H or a monovalent hydrocarbon group; and R2 and R3 are each a monovalent hydrocarbon group or the like) in such an amount as to give a molar fraction of 0.05-0.50 and structural units represented by formula II in such an amount as to give a molar fraction of 0.0001-0.50.