An antenna device includes: an antenna configured to radiate a radio wave; a substrate at which the antenna is provided; and a cover which covers the substrate from a radiation surface side of the antenna, the cover is a dielectric material, a maximum distance between the cover and the substrate in a normal direction to the substrate is smaller than ½ of a free space wavelength of the radio wave, and a thickness of the cover is smaller than ½ of an effective wavelength of the radio wave in the cover.