To easily acquire characteristic in which antenna gain, return loss, and side lobe ratio are optimum, in condition where the area of an opening part of a main reflector has been determined.

The antenna includes a main reflector 13 which radiates the microwave fed from a waveguide 11 and a slot 12, and a printed circuit board 15 arranged at an opening part of the main reflector 13. A sub-reflection pattern in which small hexagonal metal bodies 16 are arranged in honeycomb is formed on the board 15. In the sub-reflection pattern, two rows of small metal bodies 16 at a central part in magnetic field direction are arranged with a first gap Ga, and small metal bodies 16 on the outside of central part are arranged with a second gap Gb which is smaller than the first gap Ga. The antenna resonance frequency is adjusted by the gap Ga, and a return loss is optimally set by the gap Gb. The small metal bodies 16 on the edge of the opening part in the magnetic field direction of the main reflector 13 are arranged away from the opening part edge by a third gap Gc which is larger than the second gap Gb. A side lobe ratio is set by the gap Gc.