To enable the removal of a very small contaminant, which has been could not removed before, and to purify lubricating oil with higher precision.

The passage of lubricating oil feeding to an engine is branched to a main passage 3 and a bypath passage 4, a filtering body 7 with high precision is arranged in the main passage 3, and a pressure control valve 5 is arranged in the by-path passage 4. Ten to 20% of total inflow rate of the lubricating oil is flown into the main passage 3 with large passing resistance of the filtering body 7 with high precision. The pressure in the lubricating oil flowing through the main passage 3 increases, therefore 90 to 80% lubricating oil of the total inflow rate is normally divided into the bypath passage 4 by normally opening a pressure control valve 5 installed in the bypath passage. The lubricating oil B2, which does not pass through the main passage 3, is passed through the by-path passage 4, fed into the engine again so that the contaminants of the total lubricating oil B are removed by repeating circulation between the engine and the oil filter to pass through the main passage 3.