Provided is a copolymer for improving heat resistance of aromatic vinyl-vinyl cyanide based resin which can achieve preservation of excellent transparency of the aromatic vinyl-vinyl cyanide based resin, improvement in heat resistance, and a molded product having excellent appearance, by adding the copolymer to the aromatic vinyl-vinyl cyanide based resin. A copolymer for improving heat resistance of an aromatic vinyl-vinyl cyanide based resin, including: 45 to 85 mass % of an aromatic vinyl monomer unit; 5 to 45 mass % of a (meth)acrylic acid ester monomer unit; and 10 to 20 mass % of an unsaturated dicarboxylic acid anhydride monomer unit; wherein the copolymer has a total light transmittance of 88% or more, the total light transmittance being measured in accordance with ASTM D1003 for a sample with 2 mm-thickness, is provided.