PURPOSE: To keep dissolved oxygen concentration in a medium to the optimum level, by generating oxygen-enriched air by permeation through an oxygen- selective permeation membrane and controlling the supply of the oxygen-enriched air to the medium based on the detected value of the dissolved oxygen concentration in the medium.

CONSTITUTION: Dissolved oxygen concentration in medium 11 is detected with a detection means 13 during the progress of fermentation and the supply of oxygen-enriched air from an oxygen-selective permeation membrane 2 to the medium 11 is controlled by a controlling means 22 based on the detected dissolved oxygen concentration. The amount of dissolved oxygen in the medium 11 can be maintained at the optimum level and the fermentation can be smoothly carried out.