To recover phosphorus as resources from sewage and also to sharply reduce the generated quantity of sludge by adding an new idea to a conventional flocculant addition activated sludge process.

After an iron based/aluminum based phosphorus removing agent is added to a biological treatment process for biologically treating phosphorus containing organic sewage to adsorb or flocculate phosphorus, a part of sedimented sludge which a biological treatment process outflow liquid is subjected to sedimentation to form is returned to the biological treatment process, and the other part of the sedimented sludge or sludge drawn from the biological treatment process is oxidized by ozone under alkaline conditions. Next it is subjected to solid-liquid separation, and to the formed separated liquid, Ca or Mg ions are added to remove phosphorus, and then it is returned to the biological treatment process together with the separated sludge after oxidation by ozone.