NOVELTY - A sewage treatment system (1) has anaerobic tank (11) where phosphorus in activated sludge is released to natural water while taking in organic substance into activated sludge, anaerobic tank (12) where nitrite and nitrate is reduced to nitrogen gas, aerobic tanks (13a-13c) where phosphorus is taken into activated sludge while nitrifying ammonia, sedimentation tank (14) where activated sludge is precipitated, sludge return path (15) which returns the activated sludge to the anaerobic tank, digestive fluid circuit (16), load detection part (19a-19d), and natural water step flow path (20).    USE - Sewage treatment system for industrial waste water.    ADVANTAGE - Discharge of nitrous oxide gas to the atmosphere is suppressed in low cost.    DETAILED DESCRIPTION - A sewage treatment system (1) has anaerobic tank (11) where natural water supplied from natural water supply path flows in, and where phosphorus contained in activated sludge is released to natural water while taking in organic substance into activated sludge in anaerobic conditions, anaerobic tank (12) where processed natural water which flowed out of the anaerobic tank flows in, and where nitrite and nitrate contained in processed natural water is reduced to nitrogen gas in anaerobic conditions, aerobic tanks (13a-13c) arranged along the flow direction of processed natural water, where processed natural water which flowed out of the anaerobic tank flows in, and where phosphorus contained in the processed natural water is taken into activated sludge while nitrifying ammonia contained in processed water into nitrite and nitrate in aerobic conditions, sedimentation tank (14) where processed natural water which flowed out of the aerobic tank flows in, and where activated sludge contained in the processed natural water is precipitated, sludge return path (15) which returns the activated sludge deposited in the sedimentation tank to the anaerobic tank, digestive fluid circuit (16) which circulates digestive fluid in the most downstream aerobic tank to the anaerobic tank, load detection part (19a-19d) which detects ammonia load in each tanks, and natural water step flow path (20) which carries out distribution supply of a part of natural water supplied from the natural water supply path to the aerobic and anaerobic tanks based on the detection result of the nitrogen load detection part.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of the structure of sewage treatment system. (Drawing includes non-English language text).    Sewage treatment system (1)    Anaerobic tank (11)    Anaerobic tank (12)    Aerobic tanks (13a-13c)    Sedimentation tank (14)    Sludge return path (15)    Digestive fluid circuit (16)    Load detection part (19a-19d)    Natural water step flow path (20)