PURPOSE: To prevent tracking error by a side lobe by discriminating by which of a main lobe and the side lobe tracking is carried out by way of comparing an actual antenna pattern and a standard antenna pattern with each other and by controlling to carry out tracking by the main lobe.

CONSTITUTION: An antenna drive control device 4 is changed from searchin over to tracking, a tracking in a monitoring signal 9 is switching on and processing is started. Initially, tracking is carried out by an antenna 1 for a few seconds, and from angular velocity in the AZ (azimuth) direction, relative velocity of an object is calculated. Second, a control signal 8 is sent to the control device 4, the antenna 1 is swung ± several times from the tracking center in the AZ direction, and an actual receiving pattern is measured. Third, after a relative velocity equivalent amount of the object is corrected, it is compared with a reference antenna pattern, and which of a main lobe and a side lobe tracks at present is judged. In the case where it is judged that the side lobe does, by driving the antenna 1 in the main lobe direction, the object is acquired.