PURPOSE: To easily and accurately detect a multi-path by taking notice of the symmetry of an upper side wave and a lower side wave in an FM band and observing a dip on an FM spectrum generated through the multi-path.

CONSTITUTION: An IF signal is divided into two systems, and the one is used to detect a dip for the upper side wave band and the other is used to detect a dip for the lower side wave band. A multiplier 6 multiplies one system signal with a signal fed from a local oscillator 7 to apply frequency conversion. The other system signal and the output from the multiplier 6 are respectively multiplied with a chirping signal outputted from chirping signal generators 10, 11 at multipliers 8, 9. The result is subjected to band limit at band pass filters 12, 13, the signals detected by detectors 14, 15 are inputted to a comparator 16, in which a difference is extracted and the result is outputted from an output terminal 17. Thus, when a multi-path is generated, a frequency at which the dip is present thereon, the depth of the dip and the number of the dips are easily obtained.