PURPOSE: To control the amount of dissolved oxygen in the liquid in a fermentor, by controlling the speed of revolution of aeration blades placed near and under the surface of the liquid.

CONSTITUTION: A fermentor 1 is furnished with aeration blades 4 placed in the liquid phase of the culturing liquid, a cylindrical draft 5 placed above said aeration blades, stirring blades 6 placed under said aeration blades, a membrane-type oxygen electrode 9 electrically connected to a controller 10 having a DO-indicator, and an upper- and lower-limits presetter attached to said indicator. When the DO-indication is within the limits, no control ouput is transmitted. When the indication becomes lower than the lower limit, the speed of revolution of the motor 7 is increased by the signal transmitted from the controller to increase the rate of oxygen dissolution, and when the indication becomes higher than the upper limit, the speed of revolution of the motor 7 is lowered to reduce the rate of oxygen dissolution.