A filtering medium suitable for an air filter provided with (a) one or more porous membranes which include (i) high molecular weight polytetrafluoroethylene with a standard specific gravity of 2.130 to 2.230 measured according to ASTM D 4895 that can be fibrillated, (ii) a non-hot melt processable component that is not fibrillated selected from low molecular weight polytetrafluoroethylene with a number-average molecular weight of 600,000 or less; a thermosetting resin; or an inorganic filler; and (iii) a hot melt processable component with a melting point of lower than 320°C that is not fibrillated selected from a hot melt processable fluoropolymer which is selected from tetrafluoroethylene / perfluoro(alkyl vinyl ether) copolymer, tetrafluoroethylene / perfluoro(alkyl vinyl ether) / chlorotrifluoroethylene copolymer, tetrafluoroethylene / hexafluoropropylene copolymer, tetrafluoroethylene /ethylene copolymer (ETFE), tetrafluoroethylene / hexafluoropropylene /ethylene copolymer (EFEP), tetrafluoroethylene / vinylidene fluoride copolymer, tetrafluoroethylene / vinylidene fluoride / hexafluoropropylene copolymer, tetrafluoroethylene / vinylidene fluoride / chlorotrifluoroethylene copolymer, and a mixture thereof; silicone; polystyrene; polyethylene terephthalate; polyester; polyamide; or a mixture thereof; and (b) a plurality of air permeable supports which supports the porous membrane and is arranged at least on the outermost layer, wherein a pressure loss is less than 200 Pa when air is passed through at a flow rate of 5.3 cm/sec, the PF value represented by the following equation is 17 or higher when air containing NaCl particles with a particle diameter of 0.3 μm is passed through at a flow rate of 5.3 cm/sec, the dust holding capacity of polyalphaolefin particles is 20 g/m 2 or more when air containing polyalphaolefin particles with a count median diameter of 0.25 μm is continuously passed through at a flow rate of 5.3 cm/sec and the pressure loss is increased to 250 Pa, and the membrane thickness of one piece of the porous membrane is 30 um or more; PF value = − log Transmittance % / 100 / Pressure loss Pa × 1000, wherein Transmittance = 100 − Collection efficiency %. (a) one or more porous membranes which include (i) high molecular weight polytetrafluoroethylene with a standard specific gravity of 2.130 to 2.230 measured according to ASTM D 4895 that can be fibrillated, (ii) a non-hot melt processable component that is not fibrillated selected from low molecular weight polytetrafluoroethylene with a number-average molecular weight of 600,000 or less; a thermosetting resin; or an inorganic filler; and (iii) a hot melt processable component with a melting point of lower than 320°C that is not fibrillated selected from a hot melt processable fluoropolymer which is selected from tetrafluoroethylene / perfluoro(alkyl vinyl ether) copolymer, tetrafluoroethylene / perfluoro(alkyl vinyl ether) / chlorotrifluoroethylene copolymer, tetrafluoroethylene / hexafluoropropylene copolymer, tetrafluoroethylene /ethylene copolymer (ETFE), tetrafluoroethylene / hexafluoropropylene /ethylene copolymer (EFEP), tetrafluoroethylene / vinylidene fluoride copolymer, tetrafluoroethylene / vinylidene fluoride / hexafluoropropylene copolymer, tetrafluoroethylene / vinylidene fluoride / chlorotrifluoroethylene copolymer, and a mixture thereof; silicone; polystyrene; polyethylene terephthalate; polyester; polyamide; or a mixture thereof; and (i) high molecular weight polytetrafluoroethylene with a standard specific gravity of 2.130 to 2.230 measured according to ASTM D 4895 that can be fibrillated, (ii) a non-hot melt processable component that is not fibrillated selected from low molecular weight polytetrafluoroethylene with a number-average molecular weight of 600,000 or less; a thermosetting resin; or an inorganic filler; and (iii) a hot melt processable component with a melting point of lower than 320°C that is not fibrillated selected from a hot melt processable fluoropolymer which is selected from tetrafluoroethylene / perfluoro(alkyl vinyl ether) copolymer, tetrafluoroethylene / perfluoro(alkyl vinyl ether) / chlorotrifluoroethylene copolymer, tetrafluoroethylene / hexafluoropropylene copolymer, tetrafluoroethylene /ethylene copolymer (ETFE), tetrafluoroethylene / hexafluoropropylene /ethylene copolymer (EFEP), tetrafluoroethylene / vinylidene fluoride copolymer, tetrafluoroethylene / vinylidene fluoride / hexafluoropropylene copolymer, tetrafluoroethylene / vinylidene fluoride / chlorotrifluoroethylene copolymer, and a mixture thereof; silicone; polystyrene; polyethylene terephthalate; polyester; polyamide; or a mixture thereof; and (b) a plurality of air permeable supports which supports the porous membrane and is arranged at least on the outermost layer, wherein