Abstract: A process for producing a copolymer by continuously copolymerizing in a solution monomers comprising 40 to 95% by weight of an aromatic vinyl compound and 5 to 60% by weight of a vinyl cyanide compound, and further 0 to 30% by weight of a vinyl compound copolymerizable with the aromatic vinyl compound and vinyl cyanide compound, wherein 10 to 100 parts by weight of a solvent containing not less than 40% by weight of an alcohol is used as the polymerization solvent based on 100 parts by weight of the total amount of the monomers. According to the process, attachment of a gel-like polymer onto the polymerization vessel or device can be prevented. The copolymer is useful as a thermoplastic resin for various molded articles including electrical parts for domestic uses, automobile parts and commodity goods.