A fuel cap has a torque member that is attached to a casing body in a freely rotatable manner. A tether mechanism is attached to the torque member and functions to link the fuel cap with a fuel lid. The tether mechanism includes a tether rotation support that is set on the torque member in a freely rotatable manner, and a connector member. The torque member is mainly composed of a resin material, for example, polyacetal, whereas the tether rotation support is mainly composed of a thermoplastic elastomer (TPEE). The resin material of the torque member has a liquid swelling property substantially equal to or less than that of the tether rotation support. This structure of the invention ensures the good operation ability of the fuel cap even in wet weather.