PURPOSE:To obtain a granular copolymer in good efficiency, by injecting a solution of a copolymer containing at least one member selected from among maleic acid, maleic anhydride and a maleate as an essential structural unit into a nonsolvent or a poor solvent for this copolymer. CONSTITUTION:A solution of a copolymer containing maleic acid, maleic anhydride and/or a maleate as a structural unit is injected into a nonsolvent or a poor solvent for this copolymer. It is preferable that said copolymer contains at least one member selected from among isobutylene, styrene, acrylic acid, acrylates, methacrylic acid and methacrylates as structural units other than those of the maleic compounds. As said nonsolvents or poor solvents, water, 1-4C monoalcohols, 2-4C polyalcohols and 5-8C aliphatic hydrocarbons are preferable.