PURPOSE: To produce oxygen gas, easily and economically, by contacting an aqueous solution containing ferrous ion with air to oxidize the ferrous ion to ferric ion, reducing the ferric ion again to ferrous ion by electrolysis, and separating the oxygen gas generated by the electrolysis.

CONSTITUTION: An aqueous solution 3 containing ferrous ion is introduced into the absorption column 1 from its top, and made to contact with air 2 supplied from the bottom of the column 1. The ferrous ion is oxidized to ferric ion by this process according to the reaction of formula I . The reaction liquid 12 is introduced into the electrolytic cell 11, and the ferric ion is reduced to ferrous ion according to the reducing reaction of formula II. The oxygen gas 13 generated by the reducing reaction is taken out of the cell, and recycled to the receiver 6 after condensing the existing steam with a cooler 14. The liquid 12 containing ferrous ion produced by the reducing reaction is introduced into the receiver 6, heated with the heat exchanger 5, and recycled to the absorption column 1 with the pump 4.