An antenna device, including: a dielectric plate having a first surface provided with a radiation conductor and a second surface opposite to the first surface; a conductive layer provided on the dielectric plate; and a feeding structure for feeding the radiation conductor, wherein when a wavelength of a radio wave transmitted or received by the radiation conductor in air is λ, and a wavelength shortening rate by the dielectric plate is k, the conductive layer has an inner edge surrounding the radiation conductor such that a gap exists between the inner edge and an outer edge of the radiation conductor in plan view of the dielectric plate, a size of the outer edge of the radiation conductor and a size of the inner edge of the conductive layer are limited by λ and k.