Abstract: In manufacturing a thermoplastic copolymer by copolymerizing an aromatic vinyl monomer, a vinyl cyanide monomer, and, when necessary, a vinyl monomer (a) copolymerizable with the above two monomers, at least a part or all of the vinyl cyanide monomer is put in a reactor and copolymerizing reaction is started, and a weight of an unreacted portion of the aromatic vinyl monomer is caused to become greater than a weight of an unreacted portion of the vinyl cyanide monomer when all the monomer components are put in the reactor. By doing so, a ratio of the vinyl cyanide monomer in a structural unit of a thermoplastic copolymer produced at latter stages of the copolymerizing reaction is lowered. Thus, the present invention is capable of providing a thermoplastic copolymer which has excellent physical properties such as good heat resistance and good mechanical strength and is hard to discolor, as well as a manufacturing method thereof.