PURPOSE: To keep the resin extrusion rate of an extruder at the predetermined value by a method wherein the correction of the pressure setting signal and the combination of proportional and integral control action and proportional derivative control action are performed so as to control the rotation of a screw on the basis of the detected temperature and pressure of resin extruded from the extruder.

CONSTITUTION: A pressure signal and a temperature signal of a molten resin, which are detected with a pressure detector 17 and a temperature detector 30, are applied to a first and a second pressure control circuits 43 and 44 and a temperature control circuit 33. The proportional and derivative control action and the proportional and integral control action are carried out at the first and the second pressure control circuits 43 and 44. Further, in the temperature control circuit 33, the output signal corresponding to the temperature signal is applied to either one of both the pressure control circuits so as to correct its pressure setting signal, which s applied to the other pressure control circuit in order to correct its pressure setting signal. On the basis of the resultant signal, the rotational control of a screw motor 15 is done through a motor controlling 14 so as to keep the resin extrusion rate of an extruder 10 constant.