A devolatilization simulation apparatus for screw extruders and a devolatilization simulation program for screw extruders can actually simulate the devolatilization process in a screw extruder. An extruder simulation system for simulating the conditions of devolatilization of an extruder by computationally determining the resin temperature, the pressure, the residence time, the filling ratio, the torque, the power, the solid-phase occupancy and the concentration of the volatile components in terms of distribution in the axial direction of the extruder from the configuration and the operating conditions of the extruder and the physical properties of the resin in the extruder comprising a means for arithmetically simulating the process where the concentration of the volatile components contained in the resin gradually falls as they are devolatilized by vent cylinders.