PURPOSE: To obtain a resin having improved impact resistance, by extracting a rubber latex with a specific gel content and a specified particle diameter with a monomer, separating water therefrom, controlling the pressure in a plug-flow reactor of two tank-type reactor, and continuously polymerizing the monomer in a solvent under specific conditions.

CONSTITUTION: A diene rubber polymeric latex with a gel content ≥50% having an average particle diameter of 0.1W1.0μ is extracted with a monovinyl aromatic monomer, and water is separated in a water separator 5. The resultant latex is then uniformly dispersed with the monovinyl aromatic monomer and an unsaturated nitrile monomer and mixed completely in the first reactor and polymerized. The pressure of the second plug-flow type reactor is controlled by a pressure controlling mechanism 7 and kept at 1W5kg/cm2.G. The monovinyl aromatic monomer having a boiling point of -15W+5°C is present in an amount determined from the formula[X is the solvent concentration (%); p is the pressure (kg/cm2.G)]in the polymerization solution extruded from the bottom of the reactor, and the temperature of the polymerization solution is controlled at 150W180°C. Volatile components in the polymerization solution are then removed in a volatile material removing apparatus 11 to give the aimed molten polymer.