PURPOSE: To dispense with an aseptic filter and to reduce the load of a compressor, by producing an oxygen-enriching apparatus with a polymer resistant to hot water and directly connecting a culture tank with a compressor to compress oxygen-enriched gas and send the gas to the culture tank.

CONSTITUTION: The objective fermentation apparatus is composed of a blowing fan 1, an oxygen-enriching apparatus 2, a suction blower 4, a compressor 9 and a culture tank 13 and the components are connected with each other in the order. An aseptic filter placed between the compressor 9 and the culture tank 13 can be eliminated in the present apparatus in contrast with conventional process. Each part constituting the oxygen-enriching apparatus 2 is made of a polymer resistant to steam or hot water of at least ≥90°C. The oxygen- enriching membrane is preferably a 4-layered gas-separation membrane composed of a polyester substrate 17, a porous layer 18 of polysulfone or polyimide formed on the substrate 17, a silicon polymer layer 19 having crosslinked structure applied to the porous layer and a gas-separation function layer 20 composed of a crosslinked silane-grafted polymer formed on the silicone polymer layer 19.