Disclosed is a process for producing a thermoplastic copolymer, which comprises step of supplying a monomer mixture (a) to a loop reactor continuously to produce a copolymer (A), the monomer mixture (a) comprising an aromatic vinyl monomer (a1), a vinyl cyanide monomer (a2) and an N-substituted maleimide monomer (a3). The process enables the production of a thermoplastic copolymer which has an excellent balance among physical properties such as heat resistance, color and flow property, reduces the amount of bleeding that may cause mold staining during molding processing and has excellent handleability and productivity.