PURPOSE: To prevent a decrease in gain due to the aberration of a beam in a 0° or nearly 0° direction by providing an auxiliary reflector or auxiliary lens.

CONSTITUTION: A main reflector 1 and a subordinate reflector 2 reflect a nearly plane wave made incident in a specific direction so that it is converged to focal points F1 and F2. The auxiliary reflector or auxiliary lens 4 is so set that a nearly plane wave made incident in the 0° direction is converged to the center F of the phase of a corresponding horn. Horns 3a, 3b, and 3c as primary radiators face the arrival plane wave and its phase center deviates from the focal points F1 and F2, but its deviation is small and the extent of generated aberration is also small.