PURPOSE: To obtain the sufficiently mixed state of a molten thermoplastic resin and additives and to prevent the extrusion amount of a post stage extruder from becoming insufficient.

CONSTITUTION: The thermoplastic resin 4 extruded from an extruder 1 is granulated by a thermoplastic resin granulator 23. Additives 12 are injected in an extruder 11 to which the obtained thermoplastic resin particles 4b are supplied at a place where the thermoplastic resin particles 4b hold a granular shape. A first controller 28 receiving the output of a level meter 27 for detecting the output amount of the thermoplastic resin particles 4b controls the number of rotations of the screw of an extruder 1 so as to make the output from the granulator 23 constant. A second controller 30 receiving the output of a sensor 29 detecting the number of rotations of the screw 18 of the second stage extruder 11 controls the granulation speed of the granulator 23 and the number of rotations of pumps 7, 17 supplying additives 5, 12 corresponding to the number of rotations of the screw 18.