This invention is directed to a thermoplastic terpolymer excelling in thermal stability and transparency and possessing high mechanical strength, specifically a thermoplastic copolymer (I) composed of 30 to 70% by weight of (A) an aromatic vinyl compound unit, 4 to 40% by weight of (B) a vinyl cyanide unit, and 26 to 50% by weight of (C) an N-substituted maleimide and characterized by having a narrow distribution of molecular weight divisions of copolymer composition as measured by GPC, having substantially no acetone-insoluble content, possessing a weight average molecular weight in the range of 100,000 to 300,000 and a number average molecular weight in the range of 50,000 to 150,000, and having a residual N-substituted maleimide content of not more than 50 ppm. It further concerns a method for efficient production of the thermoplastic copolymer and a thermoplastic resin composition containing the copolymer I, allowing manufacture of shaped articles excelling in thermal stability and resistance to chemicals and, at the same time, possessing high impact strength, and befitting materials for automobile interior parts and electrical and electronic parts.