PURPOSE: To enable stable tracking without being affected by a radio wave in a real object, by discriminating the degree of the radiation temp. of an object with respect to a background radiation temp., and changing over the polarity of an angle error signal by using the discriminated result.

CONSTITUTION: When an antenna 1 performing a conical scanning is directed to objective matter, an object signal is obtained from a receiver 2 and a part thereof is sent to an error angle detector 3 while the remainder thereof is sent to a discriminator 5 for discriminating the degree of the radiation temp. of the object with respect to a background. Phase detection is performed in the detector 3 to output an angle error signal showing the position of the object. This signal is supplied to a polarity change-over device 4 and signal polarity is properly changed over so as to be capable of always catching the object on a viewable line by the discrimination signal from the discriminator 5 while the changed- over signal is further sent to a device to impart torque 7 through servo amplifier 6 and the antenna 1 is driven so as to minimize an angle error to perform the tracking of the object.