To provide a method for treating organic dirty water, by which phosphorus can be recovered as valuable resources, in which difficult-to-dehydrate sludge to be obtained by flocculation and settlement is not produced and which is excellent in phosphorus removing speed and phosphorus removing effect.

This method for treating the organic dirty water comprises a step to flocculate/separate SS in the organic dirty water by adding a cationic or amphoteric macromolecular flocculating agent, a step to subject the separated SS to anaerobic digestion treatment, a phosphorus removing step to adsorb/remove phosphorus by adding fine particles of iron hydroxide and/or aluminum hydroxide and the macromolecular flocculating agent to the SS-removed organic water and making the organic water thus prepared pass upward through a slurry-blanketed flocculating and settling unit in which a fluidized bed is formed, a step to subject the phosphorus- adsorbed/removed treated water to biological treatment, a step to desorb the phosphorus adsorbed on the iron hydroxide or the aluminum hydroxide contained in the slurry discharged from the flocculating and settling unit by making the slurry alkaline and recycle the phosphorus-desorbed iron hydroxide or the aluminum hydroxide to the phosphorus removing step and a step to recover phosphorus as HAP or MAP from the liquid containing the desorbed phosphorus.