An antenna device comprising: a circuit substrate including a flat conductor; and an antenna coil (21, 22, 23), the antenna coil (21, 22, 23) comprising: a plate-shaped magnetic core having first and second main surfaces; and a coil conductor wound around the magnetic core; wherein first and second conductor portions of the coil conductor are disposed in positions that do not overlap each other in a perspective plan view of the magnetic core, the first conductor portion being disposed on the first main surface of the magnetic core, the second conductor portion being disposed on the second main surface, wherein antenna coil (21, 22, 23) is disposed on the circuit substrate with the first main surface of the magnetic core and the first conductor portion facing a surface of the circuit substrate, so that a predetermined gap is made between the antenna coil (21, 22, 23) and the flat conductor, characterized in that the flat conductor is a ground electrode formed within the circuit substrate defining ground electrode formation, that the area of the flat conductor is larger than the area of the antenna coil (21, 22, 23) in a plan view of the flat conductor and the antenna coil (21, 22, 23), and that the antenna coil (21, 22, 23) is disposed so that the first conductor portion is located closer to one end portion of the flat conductor than the second conductor portion, and the entire coil conductor is located closer to the one end portion than to the other end portion in a plan view of the flat conductor and the antenna coil (21, 22, 23), that the antenna coil (21, 22, 23) is disposed on the ground electrode formation area of the circuit substrate, and that the gap between the flat conductor and the antenna coil (21, 22, 23) is set to 2mm or more. a circuit substrate including a flat conductor; and an antenna coil (21, 22, 23), the antenna coil (21, 22, 23) comprising: a plate-shaped magnetic core having first and second main surfaces; and a coil conductor wound around the magnetic core; a plate-shaped magnetic core having first and second main surfaces; and a coil conductor wound around the magnetic core; wherein first and second conductor portions of the coil conductor are disposed in positions that do not overlap each other in a perspective plan view of the magnetic core, the first conductor portion being disposed on the first main surface of the magnetic core, the second conductor portion being disposed on the second main surface, wherein antenna coil (21, 22, 23) is disposed on the circuit substrate with the first main surface of the magnetic core and the first conductor portion facing a surface of the circuit substrate, so that a predetermined gap is made between the antenna coil (21, 22, 23) and the flat conductor, characterized in that the flat conductor is a ground electrode formed within the circuit substrate defining ground electrode formation, that the area of the flat conductor is larger than the area of the antenna coil (21, 22, 23) in a plan view of the flat conductor and the antenna coil (21, 22, 23), and that the antenna coil (21, 22, 23) is disposed so that the first conductor portion is located closer to one end portion of the flat conductor than the second conductor portion, and the entire coil conductor is located closer to the one end portion than to the other end portion in a plan view of the flat conductor and the antenna coil (21, 22, 23), that the antenna coil (21, 22, 23) is disposed on the ground electrode formation area of the circuit substrate, and that the gap between the flat conductor and the antenna coil (21, 22, 23) is set to 2mm or more.