Abstract: An oil filter available for hydraulic fluid circuit of automobiles and machine tools or the like, having a bypass-discharging passage for changing an accommodated filter element for a new one, and a film which surrounds the lower end periphery of the filter element enabling pressure transmission from the hydraulic fluid toward the filter element. The bypass-discharging passage is formed in a fastening bolt which secures a casing and its cover, and when pulling off the fastening bolt, an inlet of the circulated hydraulic fluid directly communicates with a recovery passage, thereby allowing no hydraulic fluid to pass through the filter element in the casing. The film is applied so that the pressure of the hydraulic fluid may be transmitted to the outer periphery of the filter element, causing the density of the wound filter paper to be larger than the upper portion thereof.