An antenna device includes a ground conductor; a ground conductor extension that is connected to the ground conductor; and an antenna element that is connected to the ground conductor and that operates in both a first frequency band and a second frequency band higher than the first frequency band, the ground conductor and the ground conductor extension having a length that is ¼ of a wavelength of a frequency included in a middle range between the first frequency band and the second frequency band and that is not a natural number multiple of ¼ of a wavelength of the first frequency band.