PURPOSE: To increase the radiation performance, by forming the reflection mirror system incorporating the reflection mirror, sub-reflection mirror and radome and further forming the sub-reflection mirror at a part of the radome.

CONSTITUTION: The sub-reflection mirror 2 is fitted at a specified position of the radome 5. With the constitution, the support is omitted and the electric performance can be improved. Further, in deciding the shape of the main reflection mirror 1, sub-reflection mirror 2 and frame 5, the conditions of Snell's law, power and path length are satisfied and the radome 5 is handled as one element of the antenna mirror surface constitution. Thus, the electric field distribution at the opening 7 is controlled, and the electric field distribution such as the high efficiency type of uniform phase distribution, and low side lobe type of Taylor distribution can be formed.