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 (8); a rear surface cover (3) overlapping with the antenna (8) and being a conductor that faces the winding of the antenna (8); two first insulating areas (10a) provided in the rear surface cover (3) and extending in a direction that intersects the winding axis of the antenna (8); and a second insulating area (10b) that connects between the first insulating areas (10a). At least part of the area sandwiched by the first insulating areas (10a) faces the antenna (8).