A small antenna device having good communications performance and a wide communications area even when a metal plate is present in the antenna communications direction, even when the antenna is arranged, for example, inside a box-shaped metal case, and even when a through hole is used that has a smaller area than the antenna. This device comprises: the antenna; a rear surface cover overlapping with the antenna and being a conductor that faces the winding of the antenna; two first insulating areas provided in the rear surface cover and extending in a direction that intersects the winding axis of the antenna; and a second insulating area that connects between the first insulating areas. At least part of the area sandwiched by the first insulating areas faces the antenna.