PURPOSE: To avoid the aberration over entire aperture, by forming the main and subreflecting mirrors so that the optical path from the power supply horn located at the focus to the antenna aperture satisfies a given condition.

CONSTITUTION: The mirror surface and focus 1, 2 of the subreflecting mirror 3 and the main reflecting mirror 4 are constituted so that they are symmetrical to y-z plane. The aperture corresponding to the focus 2 is present symmetrically with the aperture 8 to yz plane in addition to that to the focus 1. The center of the mirror 3 is extended to the center cross sectional curve 10 present on the yz plane. One point on the curve 10 is selected and the location and slope of the point on the main reflecting mirror are obtained. Next, from the condition that the reflecting mirror 4 is symmetrical to the plane yz, the location and slope on the subreflecting mirror are obtained. Further, the location and slope of the main reflecting mirror are obtained. Further, while changing the location on the curve 10, the mirror plane system can be obtained with similar steps. Thus, no aberration is given to the antenna, since the optical path from the focus 1, 2 to the aperture 8 is constant over the entire aperture 8.