The present invention relates to an air filter unit (1) having the shape of a tube comprising a top plate (6), having an air inlet (7) leading into the interior of the tube, whereby the top plate (6) includes fasteners (13) for removably attaching said air filter unit to a wall (2) so that the air inlet of the top plate is concentrically disposed in relation to an opening in said wall, and a sealing ring (14,15) of a resilient material surrounding the air inlet in the top plate. According to the present invention, the top plate (6) includes at least one protrusion (16) on the side thereof facing said wall (2) in use, the height of said protrusion (16) being less than the distance with which the sealing ring (14,15), when not subjected to load, projects out from the surface of the top plate.