PURPOSE: To efficiently separate oil ball particles by sending water to be treated to a series of appeasement takes from a receiving tank and introducing air bubbles thereinto and rapidly floating the oil balls to form scum, and incorporating this scum into an oil collection tank and returning it to the receiving tank.

CONSTITUTION: Water to be treated is introduced into a receiving tank 10 from an opening in the upper part thereof. Continuously, the water to be treated is successively introduced into a plurality of appeasement tanks 11-13 which are arranged in series to the downstream side from the upstream side via the receiving tank 10. Air bubbles are generated by a bubble generator 4 in the appeasement tank 12 and both oil balls and sludge contg. oil incorporated in water are rapidly floated to form scum. Continuously this scum is incorporated into an oil collection tank 20 provided in the tank 13 as overflow. The overflow incorporated into the tank 20 is returned to the receiving tank 10 by a circulation pump 21. Further, water separated from oil is discharged to the outside from the bottom of the tank 13 by a drainage system 1b. As a result, the oil balls, etc., are efficiently separated by the system having a simple structure capable of being constituted of the comparatively little equipment area.