PURPOSE: To detect the deviation of a detecting region from a detectable range by a plurality of decision and processing circuits set with receivable periods different from one another and a circuit for outputting the detection of an abnormality when all the decision and processing circuits produce no detection output.
CONSTITUTION: Ultrasonic waves from a transmitting circuit 1 are reflected from an object, received by an ultrasonic oscillator 4 and inputted to a decision circuit 3 via a detection circuit 22. A receivable period is set to the circuit 3 based on the output of a gate setting circuit 12 and, when an output is obtained from the circuit 22 within the period, decision and processing circuits 31 1 and 31 2 produce the output of a level H to output units 33 1 and 33 2 , respectively. An exclusive OR circuit 32 renders an output level H during the period wherein a detection output is obtained from one of the circuits 31 1 and 31 2 and, when the output levels of both the circuits 31 1 and 31 2 is at H or L, outputs the abnormality detection signal of an output level L to the output unit 34 to cause it to generate an alarm. That is, when no reflected wave is obtained from a reference object due to changes in an environment or the like, the abnormality detection signal is outputted to indicate that a detecting region has deviated from a detectable range.