Oil separator for a crankcase ventilation system

A filter device comprises a filter element disposed in a filter housing which is to be closed by a housing cover with a sealing ring. The filter element can be supplied via an inlet in the filter housing to clean medium. On the downstream side of the filter element cleaned medium is to be discharged through an air hole in the filter housing. The housing cover is in two parts and comprises an inner lid and an outer lid, the inner lid in a housing opening and the sealing ring used between the inner lid and the wall of the housing opening is arranged. The outer cover engages over the housing opening, wherein the outer lid is connected via a closure device with the wall of the housing opening.