PURPOSE: To collect a foreign substance, e.g. water or an impurity of opposite polarity rapidly, and to remove the substance with certainty, by passing petroleum through branched ducts having electrodes to which a voltage of opposite polarity is applied, and by joining and agitating the petroleum.

CONSTITUTION: An electrical insulating liquid, e.g. petroleum, containing an emulsion of a foreign substance such as water, is injected from the inlet duct 3 into the branched ducts 4A and 4B having the electrodes 5A and 5B, to which a voltage of opposite polarity is applied, below a corona discharge starting voltage. Two kinds of charges are imparted to the emulsion substance in the liquid, and the liquid is joined and agitated in the vessel 6 to aggregate the substance with the Coulomb force. The liquid is then passed through a filter to remove the emulsion.

EFFECT: Reduced minimum aggregate emulsion diameter.