A method of treating an oil-containing waste water includes adding, where the waste water has been adjusted to have a pH between eight and twelve and have a temperature not higher than 50 C., a high-cationic flocculant consisting of a high-molecular material selected from among acrylamide-quaternized or salified dimethylaminoethyl acrylate or methacrylate copolymer, and acrylamide-quaternized or salified dimethylaminoethyl acrylate or methacrylate-acrylic acid copolymer, and which constitutes a plurality of polymerized units at least 60 mol % of which have respective cationic functional groups, to the waste water, such that a concentration of the flocculant in the waste water falls between 100 mg/L and 1,000 mg/L, so that at least one fouling component dispersed in the water is flocculated to form a sludge consisting of flocks of the at least one fouling component and thereby separate the at least one fouling component and the water.