PURPOSE: To prepare high-quality vinegar, in a short time in high yield, by vertically placing carrier membrane carrying acetobacters in a closed fermentation tank, and flowing down the raw material of vinegar along the membrane from above while keeping the atmosphere to a specific oxygen concentration.

CONSTITUTION: The top end of the carrier membrane 2 carrying acetobacter is inserted closely into the slit 4 opened to the bottom of the raw material reservoir 3 which is positioned at the upper part of the closed fermentation tank 1. The raw material of vinegar is supplied through the feed pipe 5 and the slit 4 of the reservoir 3 to the top of the carrier membrane 2, permeatd into the membrane 2, made to contact with the acetobacter in the membrane, and circulated to the tank 1 through the pipe 24, the raw material tank 11, the pump 12, the heat exchanger 16 to keep the material at the optimum fermentation temperature. Separately, the oxygen concentration in the tank is detected by the oxygen concentration sensor 9 placed in the vapor-phase, the signal of the sensor is converted to electrical output by the oxygen concentration controller 10, and the valve 7 is operated by the output signal to supply oxygen from the oxygen source 8 to the tank 1 to maintain the oxygen concentration in the vapor phase to 5W60V/V%.