To provide a cap for a fuel tank, which prevents leakage of fuel even if a container or the fuel tank is overturned.

The cap 1 for the fuel tank is for use in closing an aperture of the fuel feeding container. According to the structure of the cap, a valve mechanism is arranged across a vent hole for communicating between the inside and outside of the container 11, and when the container 11 is tilted through predetermined degrees or more, valves 4, 5 in the valve mechanism is pressed in a cap closing direction by a free sphere 6 arranged immediately below the valves, to thereby block part of the channel of the air hole.