A processing device of an organic drainage of the invention processes the organic drainage biologically in a first biological processing reaction tank 1 and processes a flotation separation for the biological processing water without agglutination in a first flotation tank 2. After processing biologically the biological processing water in the first flotation tank 2 in a second biological processing reaction tank 3, 8, a solid-liquid separation is processed. Even if processing the solid-liquid separation for micro-organisms in the processing water in the first biological processing reaction tank 1 without agglutination, if the flotation separation is adopted, the micro-organisms can be adequately removed and separated. Because of no use of agglutinant, problems that load of the organism in the biological processing reaction tank is added and the agglutinant is separated out are avoided and a device setting area and an aeration amount are reduced.