PURPOSE: To decrease load on a filter element by providing a liquid injection room which divides the upper part of the filter room, providing a liquid cleaning room in the liquid injection room so that these rooms connect at the lower part, and injecting a liquid along the tangential direction of the liquid injection room to pass through the filter element in the liquid cleaning room.

CONSTITUTION: The cutting oil sent by a pump P is injected to the liquid injection room 2a in the tangential direction, where a sludge having large specific gravity such as drain is sent to the inner wall by centrifugal force and separated from the cutting oil. The sludge falls along the inner wall to an empty can 5, where the sludge falls in a vortex flow along the inner wall of the can 5 to be collected in the lower part of the can 5. The cutting oil roughly removed of sludge, in the inside part of the initial vortex flow, forms a vortex flow and enters in the upper liquid cleaning room 2b, where the oil passes through the filter element 2 to separate sludge. During this process, the cutting oil flows in vortex in the liquid cleaning room 2b while sludge of rather large specific gravity remains by centrifugal force and falls down along the inner wall of the cleaning room 2b to the empty can 5. The cutting oil passed through the filter element 2 is returned to machine tools or the like from the liquid exit 13.