To provide a process for producing graft polymers achieving improved reaction reliability for all over the graft polymerization processes, and imparting the graft polymers with high reproducibility and good mechanical properties.

In the improved process for producing ABS-type graft polymers by carrying out an emulsion polymerization of 5-95 pts.wt. of a monomer mixture comprising (A) 50-99 wt.% of at least one vinyl aromatic monomer and (B) 1-50 wt.% of at least one comonomer, in the presence of (C) 95-5 wt.% of at least one kind of rubber base material for graft polymerization which has <10°C of glass transition temperature, the Raman-spectrum of the reaction mixture is continuously monitored and a correcting means is introduced when the concentrations of one or more monomers deviate from respectively predetermined values.