PURPOSE: To improve the capture function for impurities of fine particles by combining a circular primary filter element forming many pleats on its circumference and a circular secondary filter element formed by rolling filter paper.

CONSTITUTION: Oil introduced from an oil inlet 9 to a case 1 flows around a primary filter element 17 having many pleats 23 on its circumference and, after being filtered through the primary filter element 17 and a secondary filter element 18, is led into a pipe 3 through an opening 10 to be discharged from an oil outlet 11. The primary filter element 17 positioned on the upstream side in the oil path line captures impurities of coarse particles, while the secondary filter element 18 positioned on the downstream side in the oil path line captures impurities of fine particles of water, sludge, etc. Since the secondary filter element 18 has the structure of a multi-layered roll and the layers are closely compacted by the pressure of oil introduced, its capture function for impurities is remarkably improved.