Abstract: In internal combustion engines (1), the sealing of the combustion chamber by pistons and piston rings is not absolute. Throughout the working cycle, gases from the cylinder enter the crankcase (1a). These blow-by gases must be able to escape from the crankcase (1a). In a device for returning the blow-by gases into the system of an internal combustion engine (1) supercharged with a gas-dynamic pressure-wave supercharger (10), it is envisaged that a gas delivery unit designed as an ejector (20) generates a reduced pressure in the crankcase (1a). The ejector (20) is operated with a propellant gas from a line (16) which is taken from the supercharger airline (13), a control (8) placed in the propellant gas line (16) ensuring that the reduced pressure in the crankcase (1a) never exceeds a defined value. The blow-by gases in the line (2) flow through an oil separator (4) and, mixed with the propellant gas in the line (16), are then returned into the exhaust line (15) of the engine system.