To provide an apparatus of a diplex Doppler system for measuring the distance between cars provided with a modulating circuit part for improving distance measuring accuracy of a radar of the diplex Doppler system and reducing costs.

This radar apparatus of the diplex Doppler system is a millimeter-wave radar apparatus for computing the relative speed and the distance between one's own vehicle and a motor vehicle or an obstacle ahead by the frequencies of two digitized intermediate frequencies. The radar apparatus is provided with a means for passing an AC component of reference rectangular waves; a means for attenuating the passed AC component at a prescribed ratio; and a mean for generating a DC component for determining a central value of the attenuated AC component.