An oil separating device for an internal combustion engine connected in a flow path of blow-by gas, loaded with oil particles, from the crankcase to the inlet manifold section of the internal combustion engine and comprising an oil separator formed with at least one passage through which the flow of oil-bearing blow-by gas is guided and deflected, the oil particles being separated at a wall which causes them to flow back to the crankcase in an oil return path, is characterized in that the at least one passage is defined by a slot of small, variable slot width and large cross sectional area, the slot width being varied against spring force of an elastic member in dependence on the volume flow of the blow-by gas through the slot.