When a liquid flows into a case along a tangential direction at an arbitrary point on a side surface of a filter case from an inflow pipe provided substantially around an outer periphery, at the side surface of the filter case, a swirling flow is produced in the liquid flowed and air contained in a fluid is collected in a center portion of the swirling flow to generate bubbles. The inflow pipe and a filter element are positioned not to overlap each other in a height direction, whereby the liquid flows into a hollow portion of a filter element from a case while swirling. The bubbles generated in the center portion of the swirling flow are released to a space above a liquid surface of the fluid retained in a tank through an air outlet portion formed in an upper end surface or the side surface of the filter case.