NOVELTY - Method for treating wastewater, involves introducing the wastewater containing organic substance into a processing tank, and treating the wastewater with the activated sludge within a processing tank by adding a magnetic powder to the to-be-treated wastewater in the processing tank and causing domination by a microorganism as microbial flora, adding a micronutrient salt required by the microorganism, solid-liquid separating the magnetic powder and the sludge, and returning all or portion of the solid-liquid separated magnetic powder and sludge to the processing tank.    USE - The method is useful for treating wastewater (claimed).    ADVANTAGE - The method enables to suppress excessive increase in sludge in a biological treatment tank, reduce a discharged amount of magnetic powder discharged to outside the system along with sludge withdrawal, and treats wastewater cost-effectively and rapidly.    DETAILED DESCRIPTION - Method for treating wastewater, involves introducing the wastewater containing organic substance into a processing tank, biologically treating the wastewater with the activated sludge within a processing tank by adding a magnetic powder to the to-be-treated wastewater in the processing tank and causing domination by a microorganism belonging to Bacillus or Geobacillus as microbial flora, adding a micronutrient salt required by the microorganism in an added amount proportional to the organic matter concentration (mg/L) and inflow amount of the effluent introduced to the treatment tank or proportional to the inflow amount of the effluent, using a magnetic separation unit to cause magnetism to act on the treated water after biotreatment, solid-liquid separating the magnetic powder and the sludge, to which the magnetic powder has bonded, from the treated water using the magnetic force, and returning all or portion of the solid-liquid separated magnetic powder and sludge to which the magnetic powder has bonded, to the processing tank, where optionally the micronutrient salt is added when density (mg/l) of the mixed liquor suspended solid (MLSS) in the to-be-processed water in the processing tank increased from the steady state exceeding predetermined change rate and when the density (mg/L) of the MLSS in the to-be-processed water in the processing tank in a steady state begins to reduce.