To provide a flotation separation method for treating organic waste water by allowing minute bubbles to effectively contact the organic waste water containing a foaming component such as a surface active agent.

The method comprises a diffuser 7 disposed in a separation tank 1 to produce bubbles 11 and a self-sufficient minute bubble generator 6 disposed above the diffuser 7 to produce minute bubbles 12, allowing the minute bubbles 12 to shear off and miniaturize the bubble 11 to supply a large number of minute bubbles to the organic waste water 2. The foaming component in the organic waste water 2 rises up as bubbles 13 with the help of the minute bubbles 12 to be discharged as foam-separated water 3 while the water left in the separation tank 1 separated from the bubbles 13 is obtained as treated water 4. The amount of the air supplied to the separation tank 1 is determined on the basis of the horizontal cross sectional area of the separation tank 1.