PURPOSE: To increase partial pressure of oxygen in raw material gas efficiently and improve battery performance in an oxygen separating device by using plural number of oxygen separating towers while switching them successively.

CONSTITUTION: Air flows into from an oxygen supply pipe 1 at low pressure under a condition that valves other than the valves 3A and 4A are closed. The air is introduced into an oxygen separating tower 2A, and is brought into contact with zeolite and/or the like, and nitrogen is removed selectively. Next, before adsorbent in the tower 2A is saturated, the valves 3A and 4A are closed, and 3B and 4B are opened so as to be switched to an air separating tower 2B. Furthermore, while the nitrogen is removed in the tower 2B, the tower 2A is reactivated. In reactivation treatment, a valve 5A is opened, and the tower 2A is heated, and the separated nitrogen is exhausted from an exhaust pipe 8A by applying negative pressure thereto. After this, at the same time when the valves 3B and 4B are closed, the valves 3A and 4A are opened so as to switch from the tower 2B to the tower 2A. In this way, by repeating the switching of the towers 2A and 2B successively, oxygen having high density can be obtained continuously so that battery performance can be improved.