There is provided an antenna device for transmitting a radio wave to a tag capable of receiving the radio wave includes a first layer, a second layer, and a first plate which is disposed on or above the second layer. These are electrically conductive. The second layer is disposed apart from the first layer and includes a plurality of non-electrically conductive portions to generate an electromagnetic wave travelling along a first axis above the second layer. Further, the first plate is disposed on or above the second layer to allow the tag to receive the radio wave transmitted from the antenna.