Upon cleaning of crankcase gas generated during operation of an internal combustion engine in its crankcase, a centrifugal separator is used, which includes a rotor arranged for rotation by means of a driving motor and arranged by its rotation suck crankcase gas from the crankcase through a conduit to the centrifugal separator. During operation of the combustion engine a parameter, e.g. a measurement of the load on the combustion engine is sensed, the magnitude of said parameter being related to the amount of crankcase gas generated per unit of time in the crankcase. Depending upon a sensed change of the sensed parameter the rotational speed of the rotor of the centrifugal separator is changed in a way such that the gas pressure in the crankcase is maintained at a predetermined value, or within a predetermined pressure interval, during the operation of the combustion engine.