PURPOSE: To excellently and automatically detect a jamming radio wave, by detecting the same on the basis of the ratio of objective radio wave intensity and jamming radio wave intensity.

CONSTITUTION: The receiving signal from a preamplifier 1 is applied to a gain control amplifier 3 of an objective radio wave system and the logarithmic amplifier 6 of a jamming radio wave system through a distributor 2 and the signal corresponding to objective radio wave intensity over a wide range is formed in good linearity from a DC amplifier 10a through the gain control circuit 5 and LPF 9a controlled by a detector 4 and a gate generating circuit 14. On the other hand, a DC amplifier 10b similarily generates the signal corresponding to jamming radio wave intensity over a wide range in good linearity through the gate circuit 7, a stretcher 8 and LPF 9b controlled by the circuit 14 while the radio wave intensity ratio of both system is calculated through an adder circuit 11 and, when this ratio reaches a reference value or more, the presence of the jamming radio wave is automatically judged through a comparator 13. By this system based on the correlation of radio wave intensities of both systems, the jamming radio wave is excellently and automatically detected and the alteration and the change-over of an automatic tracking mode are properly performed.