PURPOSE: To eliminate the influence of a disturbing wave by using four sets of antennas and phase shifters and varying the frequency of a control signal which drives the phase shifters according to a prescribed function.

CONSTITUTION: A signal generated by a transmitter 5 is passed through a duplexer 6, a power divider 25, the phase shifters 19W22, and antennas 15W18 and radiated as one beam. The signal reflected by a target frame is inputted to the phase shifters 19W22 through the antennas 15W18. At this time, the phase shifters 19W22 are driven with sine wave signals which are 90° out of phase with one another and the output of the divider 25 which is composed of their outputs is passed through the duplexer 6 and mixed 9 with the output signal of a local oscillator 8 to become an IF signal, which is amplified 10 and detected 11. A modulated signal of sequential roving frequency which is led out is inputted to a phase detector 13 through a narrow band-pass filter 26 whose center frequency of its passing band is set 24 to the sequential roving frequency, phase-detected with the driving signal of the phase shifters 19 and 20, and inputted to a servo device 14 as an angle error signal. The device 14 directs the antennas so that the angle error signal becomes zero.