An air filter (1) which is intended for an air conditioning system, includes a disc type filter element (2) and a power unit (3), preferably an electric motor, which is connected to the filter element for rotating the same. The filter element is arranged in connection with a ventilation duct (4), such that it is placed outside an aperture (4a) of the ventilation duct (4) at a first distance therefrom, and such that it is concentric with the ventilation duct. The ventilation duct is supplied by way of the aperture with air (A) cleaned by the air filter. The air filter further includes a ring type collar (5), which is arranged outside the aperture of the ventilation duct at a second distance therefrom concentrically with the ventilation duct. The collar is adapted to widen towards the aperture (4a) and is most preferably in the shape of a truncated cone.