PURPOSE: To obtain the subject apparatus, designed to regulate a reactor part and separation membrane part with flow rates of circulating water and feed water, filtration pressure, etc., and excellent in controllability with stable and highly efficient separation of products by reaction with microorganisms.

CONSTITUTION: The objective apparatus constructed by installing a water level gauge 9 in a reactor 1, controlling a pump 3 for feeding water to a separation membrane 2 with the first controller 10 and controlling the first flowmeter 6 for measuring the inflow water flow rate to the reactor 1, the second flowmeter 7 for measuring the flow rate of permeated water through the separation membrane 2 and a pump 4 for feeding circulating water to the separation membrane 2 with the second controller 8.