PURPOSE: To prevent oxidation of liquid oil and to perform clarification inexpensively and continuously by allowing liq. oil in an oil tank at high temp. to permeate an intermittently moving filtration member and impressing a higher harmonic potential to the liq. oil.

CONSTITUTION: Liquid oil introduced into a filter housing 11 by a pump 30 from a high temp. oil tank 14 is let flow into a filter partition chamber 15 after permeating a filter web 21. At this stage, impurities such as scum of fried food, etc. are removed by sticking to the filter web 21, and the oil is returned while preventing oxidation to the high temp. oil tank 14 through said pump 30 and circulated. When permeation of the oil is slowed down by the deposition of impurities to the web filter 21 during circulation of the oil, a motor for a winding roll 20 for the filter is driven through a vacuum pressure relay 33 to move the filter so as to bring a fresh part of the filter web 21 to the upper surface of the filter partition chamber 15. Further, a power source potential is impressed to a higher harmonic generating device 36, and higher harmonic potential is impressed to the liq. oil in the filter housing 11.