PURPOSE: To prepare highly concentrated oxygen with a desired purity efficiently by enabling to operate in the first tower comprising three units and a secondary tower comprising two units to use general air as a stock material without pretreating the same.
CONSTITUTION: Insides of the first tower 1 of the first unit and the secondary tower 4 of the first unit are evacuated by a vacuum pump 25 to discharge a component adhered to an adsorbing agent or a wall of an adsorbing system. (desorbing operation). Subsequently, valves 8, 12 are closed and a valve 6 is opened to introduce dried oxygen with high purity from a highly concentrated oxygen sump 28 (feedback operation). At the same time, when the feedback operation in the first tower of the first unit is initiated, a valve 9 is opened and the highly concentrated oxygen of the first tower 3 is passed through the valve 9 and introduced into a part of the tower 1 through a series connecting part of a tower 4. The pretreating tower 4 is converted to the adsorbing operation and a tower 5 to the desorbing operation. As described above, even if the feedback operation is completed, introduction of a gas from the tower 3 is continued succeedingly and expelling operation of the highly concentrated oxygen in the tower 1 is carried out and the highly concentrated oxygen gas is collected at a position 28 through the valve 6 as a product.