PURPOSE: To culture microorganisms with minimum energy consumption, by controlling the amount of introducing air so as to keep the oxygen partial pressure to the level attained in the case that the minimum amount of air necessary to satisfy the maximum oxygen demand in the cultivation of the microorganisms is introduced into the system.

CONSTITUTION: The culture tank 2 containing the culture medium for microorganisms is aerated with the device 3, and the air is exhausted by the device 4. The pressure-regulation valve 6 is operated by the regulator 7 according to the signal from the manometer 5 detecting the inner pressure of the tank 2 so as to keep the pressure calculated by the pressure of the manometer 5 within the range of 1.5W3.0kg/cm2G. The regulator 10 operates the air exhaust-control valve 9 according to the signal from the oxygen partial pressure detector 8 so as to keep the oxygen partial pressure to the expected level thereof if the minimum amount of air necessary to satisfy the maximum oxygen demand in the cultivation of the microorganisms is introduced into the system.