A planar antenna includes a plurality of antenna elements and transmits and receives a radio wave to and from a target. An attitude controller is attached to the planar antenna and controls an attitude of the planar antenna mechanically. An antenna controller controls the attitude controller such that the planar antenna points in a predetermined direction with respect to the target. A scan controller controls beam scanning performed by the planar antenna and adjusts an excitation phase of each of the antenna elements in accordance with a signal level of a reception signal generated from a radio wave received from the target during performance of the beam scanning, thereby directing a beam from the planar antenna toward the target. The scan controller limits a range of the beam scanning to a range within which no grating lobe occurs.