PURPOSE: To facilitate the controlling of the rotational speed of the screw of a fore-stage extruder by a method wherein the resin pressure of a back-stage extruder is detected so as to calculate the deviation from the target resin pressure of the back-stage extruder in order to control the rotational speed of the screw of the fore-stage extruder to bring the detected resin pressure within the allowable range of the target resin pressure on the basis of the deviation signal.

CONSTITUTION: The resin pressure in the metering zone of a back-stage extruder 9 is detected with a resin pressure gage 15. The obtained detected resin pressure signal is supplied to a controller 16, which calculates the deviation between the detected resin pressure signal and the target resin pressure of the back-stage extruder 9 so as to output the rotation controlling signal of the motor 5 of a fore-stage extruder 1 on the basis of the calculated deviation in order to control the rotational speed of the motor 5 or the rotational speed of a screw 7. Since the value of deviation becomes larger when the resin pressure is detected on the back-stage extruder side and the deviation between the obtained detected resin pressure and the target resin pressure of the back-stage extruder is calculated as described above, the controlling of the rotational speed of the screw of the fore-stage extruder becomes easy. Further, by detecting the resin pressure of the back-stage extruder, the controlling of the pressure fluctuation of the back-stage extruder becomes also possible.