To make the antenna itself small with a simple structure by suppressing a side lobe sufficiently over a broad frequency band to obtain a high FB ratio.

A radiation plane 2a of a reflecting plate 2 emitting a radio wave toward an object is formed by a strip parabolic plane with a width H2 around an apex P of a paraboloid of rotation. Shield planes 2b, 2c are extended in parallel at upper and lower ridges of the radiation plane 2a in the lengthwise direction along the rotation center line L1-L1 of the paraboloid of rotation. An aperture width H1 of an aperture 3a of a primary radiator 3 is formed smaller than a width H2 of the radiation plane 2a and a width H3 between the upper and lower shield planes 2b, 2c and feeding is made to a reflecting plate 2 via a guide path. A phase center of a radio wave at an aperture 3a of the primary radiator 3 is placed on the center line L1-L1 of the paraboloid of rotation. The reflecting plate 2 and the primary radiator 3 are contained in a radome 4 and fixed integrally and driven together by the drive of a motor 12 and emit a radio wave.