This invention relates to a method for removing unreacted monomers and other volatile matters from a polymer solution composition produced from a mixture containing an aromatic vinyl monomer, utilizes two or more flash devolatilizing vessels, and effects the removal by adjusting the solid content of the polymer solution composition at the outlet of the devolitilizer before the last-stage devolitilizer to a level in excess of 97%, adding 0.5-4 parts by weight of a foaming agent to the total polymer solution composition, passing the polymer solution composition through the last-stage devolitilizer while maintaining the pressure of the polymer solution at 10 kg/cm2 or more by means of a pressure controller and the temperature in the range 190-260° C., introducing the polymer solution composition to a vapor-liquid separation vessel maintained at a vacuum of 20 Torr or less, and allowing the composition to foam.