PURPOSE: To set the front of an antenna to the top, to set the top/bottom and the right-left of a radiation amplitude pattern to be symmetric and to prevent the distortion of the radiation amplitude pattern.

CONSTITUTION: The quad-ridge horn antenna obtained by obliquely connecting a double ridge waveguide 2 to a quad-ridge waveguide 1 being a square waveguide is provided with a dielectric lens 4 at the opening part of the quad- ridge waveguide 1 and a position where the top/bottom and the right/left of the radiation amplitude pattern are set to be symmetric. in the dielectric lens 4, a dielectric constant is sequentially made higher from the opening part toward the outer side, and electric field distribution near the opening part is made uniform.