PURPOSE: To eliminate a loss of liq., to reduce the frequency of backwashings, and to enhance the concentration property of dust particles by preventing the discharge of the liq. during backwashing to the outside of the system within the limits of the possibility, and shaking off the dust particles deposited on the element surface.

CONSTITUTION: The cylindrical element 4 provided with shafts 5 and 6 on the upper and lower ends is housed freely rotatively and concentrically in a casing consisting of an upper casing 1, an intermediate casing 2, and a lower casing 3. The element 4 is rotated by the power from the outside. Then the contaminated liq. such as oil is introduced into the casing from an oil inlet 8 and filtered while passing through the element 4 from the outside to the inside, and the cleaned oil is dropped into an oil tank through the inside of the lower shaft 6. Meanwhile, backwashing is carried out by blowing the air flowing into the casing from an air inlet 9 to the outside of the element 4 from the inside, and sludge such as dust particles is discharged from the lower casing 3 as drainage.