NOVELTY - A sewage treatment system (1) contains anaerobic tank (11) which discharges phosphorus contained in activated sludge (18) to natural water, takes organic substance from activated sludge under anaerobic conditions, and allows natural water supplied from natural water supply path (17) to flow, anoxic tank (12) which reduces nitrite nitrogen and nitrate nitrogen contained in processed natural water into nitrogen gas under anoxic conditions, and aerobic tanks (13a-13c) which are arranged along flow direction of processed natural water containing phosphorus.    USE - A sewage treatment system (claimed).    ADVANTAGE - Discharge of nitrous oxide gas to atmosphere can be suppressed at low cost.    DETAILED DESCRIPTION - A sewage treatment system (1) comprises anaerobic tank (11) which discharges phosphorus contained in activated sludge (18) to natural water, takes organic substance from activated sludge under anaerobic conditions, and allows natural water supplied from natural water supply path (17) to flow, anoxic tank (12) which reduces nitrite nitrogen and nitrate nitrogen contained in processed natural water into nitrogen gas under anoxic conditions, aerobic tanks (13a-13c) which are arranged along flow direction of processed natural water containing phosphorus and nitrify ammonia nitrogen in processed natural water flowing out of anaerobic tank into nitrite nitrogen and nitrate nitrogen under aerobic conditions, sedimentation tank (14) which precipitates activated sludge contained in processed natural water flowing out of aerobic tank, sludge return path (15) which returns activated sludge deposited in sedimentation tank to anaerobic tank, digestive fluid circuit (16) which circulates digestive fluid in most downstream part of aerobic tank and anaerobic tank, and residence time control part which controls residence time of activated sludge in sedimentation tank.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of the sewage treatment system. (Drawing includes non-English language text).    Sewage treatment system (1)    Anaerobic tank (11)    Anoxic tank (12)    Aerobic tanks (13a-13c)    Sedimentation tank (14)    Sludge return path (15)    Digestive fluid circuit (16)    Natural water supply path (17)    Activated sludge (18)