PURPOSE: To prevent emergence of trouble such as accumulation of attached material on the inner wall of a bioreactor by intermittently making the interior of the bioreactor charged with microorganisms in a pressurized state.

CONSTITUTION: Plural sheets of lamellar carriers 3 having immobilized microorganisms are arranged in a tank 2 of a bioreactor 1, a raw material is sent through a line 4 to a feed opening 9 of raw material and a product is discharged from the tank. Operations for closing a solenoid valve 8, introducing a pressurized gas from an air sending line 6 to the tank to make the interior of the bioreactor in a pressurized state are intermittently carried out during fermentation.