To reduce the manufacturing cost of semiconductors, some systems have been proposed that use a cheap and high purity Clean Dry Air (CDA). CDA can reduce process step such as wafer cleaning, because CDA flow in stocker prevents the wafer surface from adsorbing of moisture and organic impurities. We have already optimized a two-stage rotary dehumidifier and have conducted a study of methods for cheaply manufacturing air that has a low dew-point of - 70 degrees C to 50 degrees C. We have further developed the method in which a dry dehumidifier is used. and developed an ultra low dew-point air generator. The air generator is a three-stage rotary dehumidifier in which a further stage is added to the two-stage rotary dehumidifier. The main component of the rotors is metal silicate. The air generator can supply dry air with a dew point of -110 degrees C. or less, in which the concentration in all gaseous contaminants is far below 1 ppb. We made a trial calculation of the manufacturing cost, and an average cost of 0.25 yen/m(3) was obtained.