An oil filter includes a casing (2) having a cylinder portion formed with an opening in one end thereof and a threaded portion formed on an inner peripheral surface of the cylinder portion, and a cap (6) threadably engaging with the threaded portion to contain a filter element between the casing (2) and the cap (6). The cap (6) includes a flange abutted on an open end surface of the cylinder portion of the casing (2) and allowing the cap (6) to be positioned by the casing (2), and a recess fittably receiving a sealing member for obstructing oil flow between the cap (6) and the casing (2). The cap (6) is formed with a facilitating portion that allows at least a part of the sealing member to move onto a radial end surface of the flange when the sealing member is mounted on an outer peripheral surface of the cap (6) at a place between the flange and the recess and the cap (6) is positioned by the casing (2).