PURPOSE: To attain an excellent VSWR characteristic in a wide frequency range and a high intersection polarization identification degree by employing a waveguide pipe loading a ridge on the waveguide converting part of a folding type primary radiant unit used as the primary radiator of a reflection mirror antenna.

CONSTITUTION: A radio wave advancing in the circular waveguide pipe 1 as a transmission line is separated into the polarization components of axes (x) and (y) by four ridges 2B symmetrically arranged, and an electromagnetic wave having a parallel polarization wave concentrates. By maintaining such a parallel polarization wave with the concentration given, the radio wave is led to the folding waveguide pipe 3. An Apex 10 and the ridge 2A are arranged in the center and on the periphery of the folding waveguide pipe so that the waveguide can be converted by holding the polarization wave separated by the ridge 2B of the transmission line, and the folded wave is emitted out of an aperture (emitting part)9.