To increase sewage-decomposing capacity in an anaerobic treatment part by effectively using treated sludge in an aerobic treatment part in the same manner as treated sludge in a solid-liquid separation part.

A sewage treatment apparatus comprises the anaerobic treatment part 1 which makes microorganisms act in inflow sewage 31 flowing in from a sewage inflow port 6 to perform anaerobic treatment for decomposing the inflow sewage 31, and discharges the treated sewage 33 from a treated sewage outlet 7, and the aerobic treatment part 2 which makes microorganisms act in the treated sewage 33 treated in the anaerobic treatment part 1, under the oxygen-supplying condition to decompose the treated sewage 33. In the aerobic treatment part 2, the treated sewage 33 from the anaerobic treatment part 1 is separated into treated sludge 33 and treated water 34, and the treated sludge 32 from the aerobic treatment part 2 is returned to the anaerobic treatment part 1.