To obtain excellent flocculation-treated water in addition to a reduction in loading amount of an inorganic flocculant in a method for loading the inorganic flocculant to carry out flocculation treatment for removing a solubilized substance and a suspended substance containing a biologically metabolic substance in biologically treated water after biologically treating organic drainage aerobically. In the method for treating the organic drainage which introduces the organic drainage into a biological treatment reactor 1 to carry out biological treatment aerobically and loads the biologically treated water flowing out from the outlet of the biological treatment reactor 1 with the inorganic flocculant, a pH of the biologically treated water at an outlet area in the biological treatment reactor 1 is made not more than 7.3.