PURPOSE: To produce high purity valuable gas with high productivity using a compact facility.

CONSTITUTION: Raw material air is sent by pressure from a raw air source 213 to a drying tower 220, and after water, unnecessary gas, etc., being removed, the air is sent to an adsorption tower 230 from a dry raw material supply pipe 252. The gas purified in the adsorption tower 230 is returned to the drying tower 220 through a recycle pipe 283 to be further purified. A part of the gas in the adsorption tower 230 is sent to the drying tower 220 through a return pipe 254 so that water and unnecessary gas, which have been a sorbed by a drying-adsorption agent 221 in the adsorption tower 230, are desorbed to be discharged from a discharge pipe 227 into the air. The purified valuable gas is sent to a reservoir tank 280 from a product recovery pipe 281 to be supplied to places where it is consumed as a product gas 290. The drying tower 220 and the adsorption tower 230 can be both pressurized and evacuated by one unit of a pump 210.