An antenna device according to an exemplary embodiment radiates electromagnetic waves. In the antenna device, a dielectric window is in contact with a lower wall of a first waveguide, the first waveguide is provided between the dielectric window and a second waveguide and extends in a direction crossing a tube axis of the second waveguide, a dispersion part in the first waveguide disperses the electromagnetic waves in the first waveguide, two inner conductors disposed at different distances from the tube axis and connected to the dielectric window include coaxial conversion parts which cause propagation of the electromagnetic waves dispersed by the dispersion part to direct to the dielectric window side, a body length of the inner conductor most distant from the tube axis, out of body lengths of the two inner conductors, is longer, and a front surface of the dielectric window does not have irregularities.