Abstract: A filter medium (10) is fashioned in a way, to effectively catch not only microparticles but also particles having nanometer dimensions. According to the invention, it comprises at least one supporting layer (20) and at least one fine fiber layer (30), the former preferably being composed of crosslinked cellulose fibers and microglass fibers, or alternatively of polymeric fibers. The diameters of the fibers in the supporting layer (20) are in the micron range and the pores of this layer mostly have cross-sections in the square micron range. The very light fine fiber layer (30) having a weight per unit area of less than 1 g/m2 possesses pores ranging down to nanometer dimensions and is thermally stable at temperatures above 180° C. It comprises fibers (40) obtained from polymers by electrostatic spinning and having diameters in the nanometer range.