Lower-limit frequency reflection characteristics of a horn antenna are improved even though element spacing, of less than or equal to one wavelength, is a spacing at which grating lobes do not occur in an antenna radiation pattern. The horn antenna includes a horn antenna and a conductor grid that divides an aperture A of the horn antenna in a grid pattern and that electrically connects to an inner surface of the horn antenna at the aperture A of the horn antenna. Width of the conductor grid in a direction orthogonal to a horn antenna aperture plane differs from electrical length of the path of the horn antenna of the conductor grid portion at the frequency of power supplied to the horn antenna.