<p>An ejector (34) is disposed on a conveying line (30) for conveying organic drain from an acid generating tank (10) to an anaerobic tank (20), biogas discharged from the anaerobic tank (20) is led to this ejector (34) by way of a gas circulating line (40) and is mixed with organice drain, and the mixture is guided to a distributor (22) provided at the lower part of the anaerobic tank (20) in a gas/liquid multi-phase flow state containing biogas as fine bubbles and is introduced into the anaerobic tank (20). Introduced biogas gives a gas lift effect to the inside of the anaerobic tank (20).</p>
