inlet side end disk 56 a radial outer element interior 64 extends between the radial outer circumferential side of the radial inner filter bellows 48 and the radial inner circumferential side of the radial outer filter bellows 46 the radial outer element interior 64 approximately has the shape of a flat oval coaxial ring space a cross section of the radial outer element interior 64 tapers approximately in a wedge shape toward the inlet side end side 52 the radial outer element interior 64 opens at the outlet side end side 54 with an outlet opening 64 for the purified air the outlet opening 66 has the shape of a coaxial ring which is flat oval in cross section the corresponding end side of the radial outer filter bellows 46 is connected leak tight at the outlet side end side 54 to a radial outer outlet side end disk 68 the corresponding end side of the radial inner filter bellows 48 is connected leak tight to a radial inner outlet side end disk 70 the outlet side end disks 68 and 70 are foamed onto the end sides of the filter bellows 46 and 48 from polyurethane similarly to the inlet side end disk 56 the outlet side end disks 68 and 70 leave the outlet opening 66 free so that purified air can flow out of the radial outer element interior 64 on the outlet side end side 54 out of the main filter element 44 the radial inner outlet side end disk 70 has an approximately flat oval shape and is closed it closes the radial inner element interior.