Provided is a vinyl alcohol-based copolymer, obtained by saponifying a copolymer of vinyl acetate and a polyfunctional monomer, wherein the polyfunctional monomer contains two or more ethylenic double bonds in its molecule, the vinyl alcohol-based copolymer has a side chain containing an ethylenic double bond and a molar ratio (d) of the ethylenic double bond to a total of vinyl alcohol units and vinyl acetate units is from 0.05/100 to 2/100, and the vinyl alcohol-based copolymer is water soluble.