PURPOSE: To improve a gain and lope and to improve an input/output impedance characteristic in a primary radiator, by constituting a sub reflecting mirror so as to form a rotational parabolic plane, a cone plane, and a cylindrical plane to a main reflecting mirror continuously in recessed shapes, and constituting a wave from the primary radiator so as to be possible to be reflected in a parallel direction to the axis of the main reflecting mirror.

CONSTITUTION: The sub reflecting mirror 2 is constituted in such a way that a point P is set as a focal point, and the rotational parabolic plane 2a is arranged on a center side, and the cone plane 2b rotating a straight line being inclined at 45° for an axis Z around the axis Z, and the cylindrical plane 2c rotating the straight line in parallel with the axis Z are arranged continuously, and the mirror is formed in the recessed shape as a whole. Thereby, all of the waves radiated from the primary radiator 3 centering the focal point P, after being reflected on the rotational parabolic plane 2a geometric-optically, become the waves advancing in a direction of X-axis, and are made incident on the cone plane 2b, and are projected on the main reflecting mirror 1 as flat waves advancing in a direction parallel with the axis Z. In such a way, since no wave reflected from the sub reflecting mirror 2 on the primary radiator 3 geometric-optically exists, the impedance characteristic of the primary radiator 1 can be improved, and also, it is possible to form an aperture plane antenna with a few shielding areas for the main reflecting mirror 1.