A fuel cap has a torque member attached to an upper portion of a casing body. The torque member is a disk-shaped plate to transmit a rotational torque applied to a handle to the casing body. An interlocking recess and interlocking claws, which constitute a plate attachment mechanism, are arranged on the upper portion of the casing member. Fitting of the interlocking claws in the interlocking recess causes the torque member to be attached to the casing body in a freely rotatable manner. This simple structure of the invention effectively prevents the fuel cap from being easily damaged by an inadvertent operation, such as a careless drop of the fuel cap, and ensures the sufficient sealing properties of the fuel cap even under application of an external load.