PURPOSE: To reliably detect an obstruction on a road surface by transmitting ultrasonic waves in a pulseform manner toward a road surface located obliquely in front and varying the amplification factor of a receiving signal based on an average value at which the receiving signal level of a reflection wave is reflected.

CONSTITUTION: Based on a command signal P1 from a timing command means 1, an ultrasonic transmitting means 3 is driven through an ultrasonic signal generating means 2 to intermittently transmit ultrasonic waves Wa-Wc to a road surface 4 located obliquely in front. Reflection waves Wa'-Wc' reflected by the road surface 4 and an obstruction 5 are received by an ultrasonic receiving means 6. After a receiving signal V2 is amplified by an amplifying means 7 and AM detection is made, averaging processing is effected by an average value computing means 8 to obtain an average value V31. From a result of comparison between the average value V31 and a given value V3R by a set means 9, the amplification factor of the amplifying means 7 is regulated by an amplification factor varying means 10, and after regulation, by comparing an amplification signal V30 with a discriminating signal V4 from a discriminating signal generating means 11, an obstruction detecting signal V5 is outputted.