An antenna device is provided that includes an antenna coil, a resin-made antenna case having a body that accommodates the antenna coil, and a rib disposed on an upper surface of the body and on a pair of opposing side surfaces. In the antenna case, the rib is continuously formed from one of the side surfaces, across the upper surface, to the other one of the side surfaces, and both ends of the rib are formed to be flush with a lower surface of the body or so as to extend downward from the lower surface of the body. In addition, both ends of the rib are designed to come into contact with an external member when the antenna device is mounted on the external member.