PURPOSE: To carry out the cultivation of aerobic microorganisms, practically without exhausting off-gas till the completion of the cultivation, preventing the frothing problem caused by a large amount of aeration, by introducing high- purity oxygen in the culture liquid of the microorganisms, and carrying out the cultivation in a closed state.

CONSTITUTION: The cultivation of aerobic microorganisms is carried out practically without exhausting off-gas, in a closed state, by introducing a high-purity oxygen having a purity of ≥95%, preferably ≥99%, into the culture system, thereby keeping the dissolved oxygen concentration in the culture liquid at ≥10ppm, preferably 15W30ppm.