A fuel cap 10 maintains high sealing even when subjected to external force; is easy to operate; and requires minimal space for accommodation. The cap device comprises a casing body 20 for closing the filler opening FNb in a sealed condition, a cover 40, a handle 45, a clutch mechanism 60 and a torque transmission mechanism 80. By upraising the handle to the handling position and applying rotational torque, rotational torque is transmitted to the casing body 20 via the clutch mechanism 60 and the torque transmission mechanism 80, closing the filler opening FNb. The handle 45 lowers back to the retracted position when released, whereupon the clutch mechanism 60 assumes non-transmission mode wherein the cover 40 and the handle 45 rotate freely if subjected to external force such as in a collision.