PURPOSE: To improve filtering ability and to enable the assured discharge of sludge, by stretching linear cathode wires against anode plates, dropping sludge adherent to the cathode wires by its own weight, and then removing the dropped sludge outside a vessel by a bucket or the like.

CONSTITUTION: A filtering vessel 1 is filled with contaminated oil, and high- voltage is charged between anode plates 8 and cathode wires 9 so that dust in the oil is let adhere to the peripheries of the cathode wires 9 by the generated static electricity. With the increase of volume, the adherent sludge is dropped by its own weight. At the bottom of the filtering vessel 1, a conveyer provided with many L-shape buckets 13 is intermittently driven to carry the dropped sludge outside the vessel by the buckets 13. The sludge adherent to the buckets 13 is removed by a brush 15 outside the vessel. The removed sludge is received in a receiving box 18. On the other hand, the purified oil is taken out from a spout 4 through a couple of support plates 5 provided with numerous small holes.