A ventilation system of an internal-combustion engine crankcase, in which the blow-by gases can be removed by bores integrated in the cylinder head and ventilation pipes connected thereto. The ventilation pipes are connected to a vacuum source such as a suction pipe, and devices pre-separate oil from the blow-by gases. At least one separate oil separation module is provided, whose oil separation chambers therein have first openings for the introduction of the blow-by gases and second openings for the return of the separated oil into the cylinder head.