PURPOSE: To obtain oxygen of high purity in the oxygen production apparatus in which oxygen is produced from the air used as the raw material with the combination of a membrane separation unit and an adsorbing separation unit by using the separating membrane having a significant difference between the transmission coefficients of O2 and Ar.

CONSTITUTION: In the oxygen production device using the air as the raw material, the membrane separation unit 10 is disposed in the previous stage of the adsorbing separation unit 20, and the separation membrane M having ≥2 transmission coefficient ratio of O2 to Ar, especially, one of silica-base membrane, cellulose acetate membrane and molecular sieve carbon membrane is used as the separation membrane M of the separation membrane unit 10. By using the separation membrane having ≥2 transmission coefficient ratio of O2 to Ar, the Ar in the enriched oxygen gas supplied to the absorbing separation unit 20 of rear stage is sharply reduced with the result that the oxygen enriched gas containing less Ar which is difficult to separate from O2 is used as the raw gas for the adsorbing separation unit. By this method, the efficiency of the adsorbing separation unit is improved and the oxygen of high purity is obtained.