PURPOSE: To improve a gain by almost uniformizing amplitude distribution at the aperture plane of a main reflecting mirror, by forming the main reflecting mirror in a rotational parabolic plane rotated around an axis in parallel with the axis including a focal point, and constituting a sub reflecting mirror of a first rotational parabolic plane arranged on a center side and setting the radiant point of a primary radiator as the focal point and a second rotational parabolic plane setting the focal point of the main reflecting mirror as the focal point.

CONSTITUTION: All of the waves radiated from the primary radiator centering the focal point P, after being reflected on the first rotational parabolic plane 2a geometric-optically, become the waves advancing in a direction of X-axis, and are made incident on the second rotational parabolic plane 2b. Since a point F is set as the focal point on the second rotational parabolic plane 2b, the wave after being reflected is projected on the main reflecting mirror 1 as if it is the wave radiated centering the point F. In such a way. it is possible to eliminate the wave reflected on the primary radiator from the sub reflecting mirror 2 geometric-optically, and simultaneously, to arrange uniformly the phase of the wave reflected from the main reflecting mirror 1 as targeted. Simultaneously, as for the amplitude distribution, it can be almost uniformized at the aperture plane, and aperture efficiency, that is the improvement of the gain can be obtained.