A fuel cap has a male threading element which is formed on an outer circumference of a closer of the fuel cap, a female threading element which is formed on an inner wall of a filler neck, and a handle. Rotation of the handle in a closing direction screws the male threading element into the female threading element. A stopper formed on the male threading element comes into contact with the start end of the female threading element to complete the closing operation. At this fully closed position, a gasket located between the closer and a sealing face of the filler neck is pressed and displaced in an axial direction to seal the sealing face of the filler neck with a sealing face pressure of not less than a preset level.