The present invention has as its object to provide an air filter not only for trapping suspended particulate substances but also for not generating hazardous contaminates in the air, which cause trouble in the manufacture of semiconductor devices. As a binder to bind fibers of the air filter together, a polymer dispersion is used in which a copolymer of a hydrophilic monomer and a hydrophobic monomer is dispersed in water. As an initiator for polymerization of the copolymer, an organic peroxide is used. When an air filter (8) is installed downstream of a chemical filter (7), the space (4) placed downstream of the air filter (8) is made substantially free of organic substances and/or inorganic substances that cause trouble in the manufacture of semiconductor devices.