PURPOSE: To prevent a malfunction due to a disturbance signal and to improve reliability by matching patterns of a transmit pulse signal and a receive pulse signal which are radiated at random with an arithmetic means, and deciding whether there is an obstacle or not.

CONSTITUTION: The detecting device consists of a transmitting means 1 which has an ultrasonic wave transmitter 4, OS oscillator 5, and timer (#2) 6, a receiv ing means which has an ultrasonic wave receiver 9 and a comparator 10, and the arithmetic means 3 which has a microcomputer 12 and a timer (#1) 16. When the timer 6 sends out a pulse signal on the basis of the command of the microcomputer 12, ultrasonic wave pulses are radiated by the ultrasonic wave transmitter 4 at random, and received by the ultrasonic wave receiver 9 and inputted to the comparator 10. Further, reflected pulses detected by the comparator 10 are inputted to the microcomputer 12 and the arithmetic means 3 matches patterns of the transmitted pulses and reflected pulses to decide whether there is an obstacle or not. Thus, even if a disturbing signal is received by the receiving means, the obstacle is detected accurately under no influence of the disturbing signal.