An antenna device includes a ground substrate including a ground electrode, and a plurality of separate substrates that are mounted on the ground substrate. Each of the plurality of separate substrates includes a dielectric substrate, and a plurality of antenna elements each of which radiates a radio wave whose polarization direction is an X-axis direction and a radio wave whose polarization direction is a Y-axis direction. The plurality of antenna elements are arranged along the X-axis direction and the Y-axis direction on the dielectric substrate on each of the plurality of separate substrates.