Outside air is cooled and dehumidified by a pre-cooler to pass through an adsorption zone of a carbon dioxide adsorbing rotor, producing air having a low carbon dioxide concentration which is cooled by an intercooler. The air that has passed through the intercooler is passed through an adsorption zone of a moisture adsorption rotor and then supplied to a low humidity working chamber. Return air from the low humidity working chamber may be mixed with the air leaving the pre-cooler. A part of the air which passed through the intercooler is branched to pass through a purge zone of the moisture adsorption rotor before being sent to a regeneration zone of the humidity adsorption rotor. Air that passed through the regeneration zone of the humidity adsorption rotor is mixed with outside air and then passed through a regeneration zone of the carbon dioxide adsorption rotor before being exhausted.