An extruder system includes an extruder for axially extruding a rubber or plastic material in a barrel. A gear pump adjusts an amount of extruded material that is discharged by engagement of gears. The gear pump includes a driving pinion and a driven pinion engaged with, and driven by, the driving pinion. A gear casing accommodates the pinions and includes side plates on both axial sides of the pinions. A casing body is arranged between the side plates and encloses a space on a radially outer side of the pinions. The side plate includes a suction side plate that has a suction port arranged opposite to that side of engagement region between the pinions and a discharge side plate that has a discharge port arranged opposite to that side of engagement region between the pinions. The suction and discharge ports are maintained out of axial communication with each other.