A non-return valve for an oil return in the crankcase ventilation system of a combustion engine includes a valve housing, a valve body which is mounted in the valve housing so as to be displaceable as a whole between an open position and a closed position, wherein the valve body in the non-operating state of the combustion engine is maintained in the open position without preload and the valve body is configured to automatically move to the closed position in case of an overpressure at the crankcase side with respect to the oil separator chamber. The non-return valve is provided and configured to be inserted into a mounting of a cylinder head cover of the combustion engine.