A filter medium includes at least a first and a second layer. Both first and second layers have a layer volume VLq and a layer filtering surface SLq, where SLq is the accumulation of all mantle surfaces of the fibers present in this layer q. The first layer has a SL1/VL1-ratio of less than 32500 m.sup.2/m.sup.3, and the second layer has a SL2/VL2-ratio being more than 1.1.times.SL1/VL1.