PURPOSE: To detect automatically the submergence state of a road or the like, by measuring the time required for arrival of ultrasonic waves, which are transmitted from an ultrasonic wave transmitter, at a wave receiver and comparing this time with a preliminarily set reference time.

CONSTITUTION: An ultrasonic wave transmitter 2 and an ultrasonic wave receiver 3 are installed on a road side 1. An AND circuit 7 receives a signal (a) from an ultrasonic oscillating circuit 6 and transmitted pulses (b) from a counter 10 and leads out an intermittent ultrasonic signal (c). The signal (c) is amplified by an amplifying circuit 8 and is transmitted as a signal (d) from the ultrasonic wave transmitter 2. When the road is submerged, the signal (d) is propagated in water and the speed is higher, and therefore, the time required for obtaining a wave receiving signal (e) is very short, and the width of a set output (i) of an FF circuit 14 is reduced. Therefore, the counted value of a counter 17 which counts clock pulses from an AND circuit 16 is very small and becomes smaller than a reference time value, and A<B is true in a comparator 18, and a high- level signal is led out, and thus, the submergence state of the road is detected.