A sewage treatment system is provided, for cleaning a sewage by filtering solid components in the swage through floating filter medium, wherein the filtration performance can be improved and the filtration cleaning can be performed efficiently. The sewage (18) is flowed into the treatment tank (12) in upward-flow, and filtered through the floating filter media layer (22) made of the meshy cylindrical floating filter medium (20) which have smaller specific gravity than the sewage (18). To wash floating filter medium (20), the air is jetted from the air jetting pipe (54) and the revolution flow is generated to scale solid components adhering to filter medium, and then the sewage (18), which has washed the floating filter medium (20), are discharged to the outside of the tank (12) while the air is jetted from the air jetting pipe (56) for full jetting to jet the floating filter media layer (22) in whole.