PURPOSE: To obtain good detecting sensibility even in case of the reflection factor of a road surface to be detected being not so much different from the ordinary road surface by detecting the surface state which using the reflected wave inputted after repeating the reflection between the surface and the lower face of a car body as the measuring object.

CONSTITUTION: A delay circuit 16 is driven by a timer at the signal rise time of the trigger signal (b) outputted from a trigger signal oscillator 6 and outputs a delay signal (f) after the prescribed time TD. This time TD is set at the slightly smaller in value than the predicting time when the (n)th reflecting wave arrives after the signal (b) to be decided by the distance between the surface and vehicle being oscillated. On the other hand a sample and hold circuit 15 is actuated after the inputting of the signal (f) and inputs the envelope of the (n)th reflecting wave Sn from an envelope follower circuit 10 to compare the mean intensity, for instance, of the detected wave and the reference intensity of the (n)th reflecting wave. And if the detected wave intensity be smaller than the reference intensity it is judged that the detected surface L is a snow road, for instance, and the inner part counter is counted up by one. A statistics processing circuit 12 outputs a snow road detecting signal when the number of times of counting is more than the prescribed times.