An antenna device includes a ground plate providing a ground potential, a first feeding part and a second feeding part provided to the ground plate, a vertical antenna element electrically connected to the first feeding part, spaced apart from a first surface of the ground plate, and configured to emit a radio wave having a polarization plane in a direction perpendicular to the ground plate, a horizontal antenna element electrically connected to the second feeding part, arranged in parallel with the ground plate, and configured to emit a radio wave having a polarization plane in a direction parallel to the ground plate, and an antenna base disposed on a second surface of the ground plate, and facing the ground plate and the horizontal antenna element. In the antenna base, at least a portion facing the horizontal antenna element is a dielectric.