A crankcase breathing system (1) for an internal combustion engine includes at least one oil separating apparatus with a preliminary separator (3) and a main separator (4) arranged downstream of the same, with the preliminary separator (3) including a diffuser (11) between an inlet (7) and an outlet (8) which expands in the direction of flow. In order to achieve high separation rates in the simplest possible way it is provided that the preliminary separator (3) is arranged integrally with the main separator (4), with preferably the preliminary separator (3) and the main separator (4) forming a separator unit (2).