A cap device that closes a tank opening engages with an opening engagement element formed in a periphery of a tank opening. The cap device includes a handle mechanism that is rotationally mounted on a closer. The handle mechanism rotates the closer to close and open the tank opening. A torque mechanism is located between the handle mechanism and the closer and selectively transmits torque in the closing direction and the opening direction to the closer. The torque mechanism is constructed to prevent an engagement sound from being produced when the tank is being opened.