To efficiently treat sewage by easily keep the water level of a nitrifying treatment part 3 within a set range.

A main inflow passage 7 including a weir 9 for adjusting an inflow opening area, and a sub-inflow passage 8 including a pump 11 for adjusting an inflow amount in response to the water level are mutually connected in parallel to the nitrifying treatment part 3. A main outflow passage 14 including a weir 16 for adjusting an outflow opening area, and a sub-outflow passage 15 including a pump 18 for adjusting an outflow amount in response to the water level are also mutually connected in parallel to the part 3. Thus, a flow rate is adjusted in the sub-inflow passage 8 and the sub-outflow passage 15, in addition to the adjustment of the flow rate in the main inflow passage 7 and the main outflow passage 14.