A fuel cap has a torque mechanism that transmits a rotational torque to a casing main body through engagement elements in response to rotation of a cover. When the rotational torque exceeds a predetermined level, the engagement is released to give a click. The first torque arm is constructed as a cantilever beam having a free end facing an arm support member across a preset gap. Under application of pressure, the first torque arm is deformed and transmits a rotational torque in the closing direction to the casing main body. This arrangement assures setting of the rotational torque of the torque mechanism to the predetermined level to apply a sufficient sealing stress to the fuel cap.