An antenna device that includes coil antennas each having a coil conductor wound around a winding axis and a planar conductor including a surface and an edge end portion, the surface extending along the winding axis of the coil conductor, the edge end portion being adjacent to a coil opening of the coil conductor. A current flowing through the coil conductor induces a current in the planar conductor, this current produces a magnetic flux in a direction normal to the planar conductor, and thus the planar conductor acts as a booster antenna. The antenna device has directivity in the direction normal to the planar conductor because the magnetic flux produced by the coil antennas and that produced by the planar conductor are combined. This enables the antenna device to occupy a small area while achieving a predetermined communication distance.