To obtain a powdered product having a reduced amount of a residual solvent without imposing a heavy load on drying equipment in removing the organic solvent from a viscous maleic anhydride-alkyl vinyl ether copolymer solution obtained by solution polymerization.

From a slurry solution obtained by adding a poor solvent to the copolymer solution to deposit the copolymer, the solvent is removed by equipment such as a drying machine, a granulator or the like.