PURPOSE: To efficiently perform purification in the min. number of tanks, by packing a tank body with contact bodies rich in a microorganism on the grid plate in said tank body so as to reach the upper part thereof, and covering the tank body with a perforated tank lid wherein ground opening cylinders provided with lids are provided directly above a flow-down pipe and a rising pipe in a protruded state.

CONSTITUTION: Inflow sewage from an inflow port 3 rises while receives an inflow port 3 rises while receives the decomposition action of an org. substance due to a microorganism in a tank while the height increased portion thereof is flowed into a flooding pipe 8 through water collecting orifices 9 and fallen through an introducing pipe 7. The fallen water is raised by an air lift pump 12 and returned to a flow-down pipe 5 through a recirculation flow-down pipe 11. The sewage is recirculated several times in the tank from inflow to outflow and the purification degree of the sewage at the outflow port is enhanced. The components slow in decomposition within the org. substance are precipitated to the bottom of the tank to be converted to sludge which in turn obstruct the recirculation of the sewage and, therefore, said sludge is raised by the air lift pump 12 provided to a position where sludge is easy to be rotained and returned to the flow-down pipe 5 by a return pipe 13.