A cap device of the invention has a tether mechanism, which is used to link a fuel cap to a fueling lid. The tether mechanism includes a cord-like flexible linkage body having one end as a first support end, which is fit in a support member of a rotating ring. The first support end is inserted from inside of the rotating ring through an insertion opening into a support recess. This fits the first support end in the support recess, while allowing the linkage body to be led through a lead opening. The first support end is accordingly fixed to the support member. The rotating ring is then attached to a circular groove formed on the fuel cap. The tether mechanism of this simple structure is readily fixed and thereby ensures easy attachment to the fuel cap.