Cap device

A fuel cap 10 comprises a casing body 20, a handle mechanism 40, a clutch mechanism 60, and a torque mechanism 80. The clutch mechanism 60 comprises a clutch switch 61 in the form of a cantilever which is pushed during the installation operation, and a one-way clutch unit. Squeezing an operating portion 47 and operating in the closing direction causes the clutch switch 61 to flex, maintaining a clutch arms 62 and clutch teeth 64 in the engaged state so that rotational torque is transmitted. The fuel cap 10 maintains a good seal even if the handle mechanism 40 should be subjected to external force, while at the same time providing superior ease of operation during installation and removal.