PURPOSE: To clean up efficiently waste water by removing fine oil components therefrom by providing an electrical tank for separating oil-components constituted in such a way that the inside is maintained in the aeration atmosphere occuring in generation of foam in the required stage of a series of cleaning up stages.

CONSTITUTION: ⊕ ions and ions are generated in waste water by the potentials of the ⊕ plate and plate of individual electrode plates 6, 6 through which electric current is passed. The electron which is the outside layer of the Helmholtz's double layers in an oil drop (a) and the ⊕ ion in the water are neutralyzed by charge and the ⊕ charge of the oil drop (a) and the ion in the water are neutralyzed by charge. The zeta potential attains therefore zero, i.e., an isoelectric point and the micro oil drop (a) ascends together with water. The drop (a) is separated from the water at the water surface and is taken out via a scum removing tray 5.