An extruder system with a gear pump, comprising an extruder for axially extruding a rubber or plastic material in a barrel by rotation of an extrusion screw arranged in the barrel, and a gear pump driven by a rotational force for rotating the extrusion screw so that a predetermined amount of the material extruded from the extruder is discharged by engagement of gears, wherein: the gear pump comprises a driving pinion arranged coaxially to the extrusion screw and fixedly secured to a tip end of the extrusion screw, at least one driven pinion meshed with, and driven by the driving pinion and rotatable about a rotational shaft that is parallel to a rotational shaft of the extrusion screw, and a gear casing housing the driving pinion and the at least one driven pinion; the gear casing comprises side plates (12,13) arranged on both axial sides of the driving pinion and the at least one driven pinion leaving a small clearance from the driving pinion and the at least one driven pinion, and extending perpendicularly to the rotational shaft of the extrusion screw, and a casing body arranged between the side plates (12,13) and enclosing a space on a radially outer side of the driving pinion and the at least one driven pinion 5; the rotational shaft of the at least one driven pinion is fixedly secured to the side plates (12,13); and the side plates (12,13) includes a side plate situated on the suction side adjacent to the extrusion screw and a side plate situated on the discharge side remote from the extrusion screw, the side plate on the suction side having a suction port that is arranged opposite to that side of an engagement region between the driving pinion and the at least one driven pinion, where teeth of the driving pinion and the at least one driven pinion are being disengaged from each other, and the side plate on the discharge side having a discharge port that is arranged opposite to that side of the engagement region between the driving pinion and the at least one driven pinion, where teeth of the driving pinion and the at least one driven pinion are being engaged from each other, the suction port and said discharge port being maintained out of an axial communication with each other by the driving pinion and the at least one driven pinion. the gear pump comprises a driving pinion arranged coaxially to the extrusion screw and fixedly secured to a tip end of the extrusion screw, at least one driven pinion meshed with, and driven by the driving pinion and rotatable about a rotational shaft that is parallel to a rotational shaft of the extrusion screw, and a gear casing housing the driving pinion and the at least one driven pinion; a driving pinion arranged coaxially to the extrusion screw and fixedly secured to a tip end of the extrusion screw, at least one driven pinion meshed with, and driven by the driving pinion and rotatable about a rotational shaft that is parallel to a rotational shaft of the extrusion screw, and a gear casing housing the driving pinion and the at least one driven pinion; the gear casing comprises side plates (12,13) arranged on both axial sides of the driving pinion and the at least one driven pinion leaving a small clearance from the driving pinion and the at least one driven pinion, and extending perpendicularly to the rotational shaft of the extrusion screw, and a casing body arranged between the side plates (12,13) and enclosing a space on a radially outer side of the driving pinion and the at least one driven pinion 5; the rotational shaft of the at least one driven pinion is fixedly secured to the side plates (12,13); and side plates (12,13) arranged on both axial sides of the driving pinion and the at least one driven pinion leaving a small clearance from the driving pinion and the at least one driven pinion, and extending perpendicularly to the rotational shaft of the extrusion screw, and a casing body arranged between the side plates (12,13) and enclosing a space on a radially outer side of the driving pinion and the at least one driven pinion 5; the rotational shaft of the at least one driven pinion is fixedly secured to the side plates (12,13); and the side plates (12,13) includes a side plate situated on the suction side adjacent to the extrusion screw and a side plate situated on the discharge side remote from the extrusion screw, the side plate on the suction side having a suction port that is arranged opposite to that side of an engagement region between the driving pinion and the at least one driven pinion, where teeth of the driving pinion and the at least one driven pinion are being disengaged from each other, and the side plate on the discharge side having a discharge port that is arranged opposite to that side of the engagement region between the driving pinion and the at least one driven pinion, where teeth of the driving pinion and the at least one driven pinion are being engaged from each other, the suction port and said discharge port being maintained out of an axial communication with each other by the driving pinion and the at least one driven pinion.