An antenna device includes at least one antenna conductor, at least one ground conductor, and an artificial magnetic conductor that is located between the at least one antenna conductor and the at least one ground conductor and is disposed separately from the at least one antenna conductor and the at least one ground conductor. At least one of the artificial magnetic conductor and the at least one ground conductor includes at least one opening formed at a place substantially facing a distal-side end of the at least one antenna conductor, the distal-side end of the at least one antenna conductor being opposite a feeder-side end of the at least one antenna conductor.