To prevent the adhesion of the crest parts of filter paper to each other in the use of a filter element in which the pleated filter paper is rounded into an annulus ring.

The diameter of the end part of an inner cylinder 17 to be put into a filter element 4 is decreased, and the position of a valve holding part 17c at the tip of small-diameter part 17b is arranged to be inside sufficiently from the position of an oil inflow hole 10. The diameter D of the general part 17a of the inner cylinder 17 and the inside diameter of the filter element 4 are enlarged in relation to the diameter (d) of the part 17b to prevent the adhesion of the crest parts of filter paper which forms the filter element 4 to each other.