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 . 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 .