Abstract: A fuel cap is provided to improve sealing characteristics of a pressurized fuel tank. When the inner pressure P of a fuel tank is a negative pressure less than a predetermined regulation negative pressure Pr which does not need to be controlled, a valve 160 is pressed upward by the urging force of a spring 164 to bring a lip portion 161 of the valve 160 into contact with a seal surface 156 of a seal body 150 to thereby close the valve 160. As a result, the fuel tank is sealed from the outside air. At this time, a flat plate portion 155 of the rubber-made seal body 150 is bent by the urging force of the spring 164 so that the gap L between the flat plate portion 155 and the lower surface of a ceiling plate portion 131 is narrowed. In addition, the divergence of center of the valve 160 with respect to the seal body 50 is limited by ribs 162 of the valve 160 located in a valve slide hole 135.