An air filter unit in which the generation of organic substances is restrained by controlling at least the amount of organic substances detected from a square test piece of 5.times.5 cm (25 cm.sup.2) of a filter medium so as to be 100 ng or less in the total amount of organic substances detected from the whole air filter unit at a temperature of 40.degree. C. before using. The air filter unit is assembled by housing a filter medium that has been bent in a waveform in a frame composed of 4 outer frame members and sealing the gap between the frame and the filter medium using a sealing compound. Organic substances adhering to the air filter unit are removed by blowing a cleaned air heated at 80.degree. C. against the air filter unit at a velocity of 50 ml/min for 60 minutes. Then, an air suction port and an air exhaust port of the air filter unit are sealed with a film (for example, a PET film) which keeps the unit from contacting with organic substances and from which organic substances are not generated.