PURPOSE: To make humidity of a dehumidified air constant, independently of the change of a consumption amount of air by providing a sensor for detecting the humidity of the dehumidified air, and by controlling a flow rate controlling value receiving a signal from the sensor in such way that the humidity of the dehumidified air is constant.

CONSTITUTION: The air is dehumidified at the time of going out of a module 3, and the humidity (hereinafter referred to as dew point) of the dehumidified air is detected with the sensor 8 which detects the humidity of the dehumidified air and is provided to a conduit 5 for the dehumidified air. And, when said air does not reach a prescribed dew point, the signal is sent to a controlling means 9, and the dehumidified air which is reduced to the atmosphere pressure with a reducing value 7 is recycled to the permeation side of the module 3 through a return pipe 6 as a carrier gas. At this time, the dew point of the dehumidified air is controlled to a prescribed value by controlling the opening of the flow rate controlling valve 10 provided to the return pipe 6. The dehumidified air is compressed and stored in a pressure tank 11, maintaining said air at the prescribed dew point.