To constitute a fuel cap 20 with a simple mechanism, the cap having a holding mechanism 70 that temporarily holds itself when it is removed from the inlet opening, while ensuring the cap's having its inserting capability not deteriorated when re-inserted into the inlet opening FNb.

A cap device includes a holding mechanism 70 that holds the cap temporarily when it is removed from the inlet opening FNb. The holding mechanism 70 is composed of an attachment groove 72 at the lower part of the fuel cap 20 and of a cap holder 74. A smoothly convexed insertion guide section 73 is installed around the outer circumference of the fuel cap 20 toward the upper surface of the attachment groove 72, thus the attachment groove 72 is found difficult to be caught by the inlet opening FNb, resulting in the cap's not having its inserting capability deteriorated.