PURPOSE: To collect dust in an oil and remove the dust therefrom continuously and efficiently by employing a plurality of groups of insulating materials which are provided between a high potential electrode and a grounded electrode such that the insulating materials are extended in parallel with a current of oil and such that one group of the insulating materials are opposed to the other in a staggered manner, whereby dust in the oil can be collected in the vicinity of the electrode surfaces.

CONSTITUTION: Insulating plates 23 are disposed in contact with the inner surfaces of a high potential electrode 5 and a grounded electrode 6, which are provided in a metal casing 10. Two in sulating plates 21 are provided in the casing 10 in an opposed manner such that the distance between the insulating plates 21 is equal to the inner diameter of an end portion, into which an oil is discharged, of a pipe 20 and such that the insulating plates 21 are staggered at their free end portions with respect to the insulating plates 23. When a DC high voltage is applied to the high potential electrode 5, electric potentials are distributed in accordance with equipotential lines 14 shown in the drawing. When the voltage applied is positive, negatively charged dust among the dust flowing with an insulation oil 4 is moved in the direction of an arrow 12, and positively charged dust 1 in the direction of an arrow 30. The dust 7, 2 is collected in a collector portion 27. The purified oil is discharged through the pipe 20.