A cap device or fuel cap has a cap main body, a cover, and a torque plate that transmits a rotational torque. The torque plate is attached to the cap main body in a rotatable manner through engagement of catching claws formed on the outer circumference of the torque plate with catching projections formed on the cap main body. The cover is held on the torque plate in a rotatable manner through engagement of engagement projections formed on the inner wall of the cover with catching claws formed on respective fixation elements. The engagement projections are arranged about an axis of the cap main body and outside the catching claws. The engagement projections are thus readily released from engagement with the catching claws under application of an external force in any of diverse directions, for example, due to a collision of a vehicle, so as to maintain the sufficient sealing properties of the cap main body.