A fuel cap 1 for controlling the pressure within a fuel tank which has improved sealing properties between a valve member 43 of a pressure control valve 41 and a seat surface 35. The pressure control valve 41 opens and closes a flow path hole 33 in a cap body 5 so as to control the pressure within the fuel tank. In this pressure control valve, the valve member 43 is seated on the seat surface 35 in accordance with the force of a spring 71 and the pressure within the fuel tank. The valve member has a disk portion 49 which is flexed when the valve member receives a large pressure in a valve closing direction. An outer lip 57 and an inner lip 58 are formed on the disk portion 49. The outer lip 57 is seated on the seat surface 35 when the pressure within the fuel tank reaches a predetermined pressure. When the pressure increases further, the disk portion 49 is flexed, so that the inner lip 58 is also seated on the seat surface 35. Therefore, when the pressure in the valve closing direction becomes great, the double lip portion is seated on the seat surface 35, thereby providing enhanced sealing properties.