The invention provides a drain mechanism of a fluid filter in which a heated fluid is not splashed on a worker at a time of discharging the fluid such as a time of exchanging an element, and can inhibit dusts or the like from being mixed from an external portion at a time of normal use. A drain mechanism 7 of the present invention is provided with a cap 3 engaged with a case 2 and having a drain hole 15, a drain member (a drain plug 20) detachably mounted to the cap from an outer side thereof and closing the drain hole, a valve member 25 provided in an inner side of the cap and closing the drain hole, and an elastic means (a spring 11) for energizing the valve member in a direction of closing the drain hole. In a state in which the drain member is detached from the cap, a residual fluid within a housing is discharged by a tubular draining jig 30 inserted to the drain hole.