PURPOSE: To provide a feed horn for a circularly polarized wave suitable for miniaturization, with an excellent cross polarized wave characteristic and offering ease of connection to an LNB.

CONSTITUTION: The horn is made up of a radio wave output means comprising a dielectric plate 3 and a conductor pattern 4 provided to an opening 1 of a circular waveguide 2, a parallel grating pattern 6 provided to a dielectric plate 5 located at a distance equivalent to a nearly 1/4 wavelength of a radio wave propagated in a circular waveguide 1 from said output means and a radio wave reflection means provided at a distance equivalent to a nearly 1/8 wavelength of a radio wave from the parallel grating pattern 6 and comprising a dielectric plate 7 and a copper foil face 8, and a polarized wave component whose phase is delayed more than that of the other in two orthogonal polarized wave components of a circularly polarized wave introducing to the circular waveguide 2 is reflected in the parallel grating pattern 6 and the polarized wave component whose phase is led is reflected in the copper foil face 8 and converted into a linearly polarized wave at a position of the output means and the converted wave is outputted.