PURPOSE: To inhibit decrease in the concentration of dissolved oxygen (DO) at a low cost without practically requiring power expenses, by providing a rotor for controlling a flow speed and a detector for detecting the amount of dissolved oxygen in the passageway of an endless water conduit and a buffle plate at the specified position of said endless water conduit, respectively.

CONSTITUTION: Inflow water is supplied from the outside to a water conduit 1, while activated sludge is further supplied from the outside. A buffle plate 4 is provided in the passageway of the water conduit 1, and a detector 5 for detecting the amount of dissolved oxygen is provided at the outlet side of the buffle plate 4. Said detector 5 for detecting the amount of dissolved oxygen measures the concentration of oxygen in the inflow water inside the passageway, to control the buffle plate 4 at a position acting as an obstacle against the passageway of the water conduit when the measured value is below the reference amount of dissolved oxygen. Hence, bubbles are formed to disturb the passageway by the appearance of the obstacle in the passageway, so that the oxygen is mixed in the inflow water. On the other hand, when the measured value is above the reference amount of dissolved oxygen, the buffle plate 4 is made to hold its posture as it is.