PURPOSE: To obtain a complicated mirror face shape with one process by dividing a mirror surface of a reflecting mirror is lattices and optimizing directly a shift of each lattice point.

CONSTITUTION: A mirror surface of a reflecting mirror and a main reflecting mirror is divided into lattices, and the deviation in the mirror surface at the lattice point is directly optimized by minimizing the difference between the far field bound gain and the antenna gain. The antenna is made up of, e.g. 6 feeding horns and one reflecting mirror 1. When the reflecting mirror 1 is a parabolic mirror, 6 circular beams are reflected in the reflecting mirror 1, and e.g. a radiation pattern is overlapped on a Japanese map when viewing from a still orbit at the 110th degree of the east longitude. In order to improve the radiation pattern to be matched more to the geography of Japan, the reflecting mirror 1 is corrected. In this case, the mirror surface is adjusted by dividing an opening face 3 into N-sets of lattices and moving in parallel a point on the mirror surface with respect to each lattice pint by a deviation ΔZ in the Z axis direction. Thus, the complicated mirror face shape is obtained by one process.