A feed probe (1), a semicylindrical sub-reflector (2) forming a primary radiator together with the feed probe (1), and a main reflector (3) arranged such that mirror surfaces of said main reflector (3) and the sub-reflector face across the feed probe (1) are all disposed on a ground plate (a recfector face) (4). The main reflector (3) has a predetermined focal point or focal line on which the feed probe (1) is located, and is mounted on the ground plate (4) at a predetermined installation angle θ. A converter (500) for converting linearly and circularly polarized waves is provided on the mirror surface of the main reflector (3). The converter (500) is composed of a plurality of grooves (510) and ridges (512) formed between the grooves, so that a wave component orthogonal to the grooves is reflected at the bottom of the grooves while a wave component parallel to the grooves is reflected on the ridge surface, thereby causing a phase difference according to the height H of the grooves when a radio wave is reflected on the main reflector to thereby perform linear to circular polarization conversion.