PURPOSE: To contrive the improvement of a gain and side lope and the improvement of an input/output impedance characteristic in a primary radiator, by forming a main reflecting mirror and a sub reflecting mirror as rotational parabolic planes rotated around the same axis, and constituting the primary radiator in such a way that it has an aperture diameter almost equal to the diameter of the sub reflecting mirror and it projects a flat wave on the sub reflecting mirror.

CONSTITUTION: A wave surface immediately after being radiated from the primary radiator 3 is maintained at an aperture part, and is radiated as the flat wave advancing almost in a direction of axis Z. And when the flat wave is made incident on the sub reflecting mirror 2, the wave, after being reflected on the sub reflecting mirror 2 geometric-optically, is focused on the focal point F of the rotational parabolic plane, and afterwards, it is radiated centering the focal point F, and is projected on the main reflecting mirror 1. Since the main reflecting mirror 1 sets the point F as the focal point, and it is the rotational plane of a parabola setting its axis as an axis Z' in parallel with an axis Z, all of the waves from the focal point F are radiated from an antenna as the flat waves advancing in the direction of axis Z. Therefore, it is possible to almost uniformize amplitude distribution at the aperture plane of the main reflecting mirror, then, to improve the gain. and also, to improve the impedance characteristic of the primary radiator by reducing a reflected wave on the primary radiator.