A fuel cap of the invention has a pressure regulating valve, which includes a positive pressure valve and a negative pressure valve. The positive pressure valve has a positive pressure valve plug and a first spring. The negative pressure valve has a negative pressure valve plug and a second spring. A ring-shaped projection is protruded outward from the upper circumferential end of the negative pressure valve plug to face a first valve flow path, and functions as a restriction member. The restriction member narrows the passage area of the first valve flow path in a closed position of the negative pressure valve plug, while widening the passage area of the first valve flow path in an open position of the positive pressure valve plug where the negative pressure valve plug moves integrally with the positive pressure valve plug. This arrangement effectively improves the flow characteristics of the pressure regulating valve in the open position of the positive pressure valve.