A sewage treatment system is provided for cleaning sewage by filtering solid components in the sewage through use of a floating filter medium, wherein the filtration performance can be improved and the filtration cleaning can be performed efficiently. The sewage flows upwardly in the treatment tank and is filtered through the floating filter media layer made of cylindrical mesh floating filter media which have a smaller specific gravity than the sewage. To wash the floating filter medium, air is jetted from the air jetting pipes to generate a circular flow to scape off solid components adhering to the filter medium. Then the waste, which has washed the floating filter medium is discharged to the outside of the treatment tank while the air is jetted from the air jetting pipe for full jetting of the floating filter media layer unidirectionally.