PURPOSE:To improve a noise removal rate by judging only a wave which has small pulse variation in a received signal to be a reflected wave. CONSTITUTION:An ultrasonic wave which is sent by an ultrasonic wave sender 4 and reflected by an obstacle 0 is received by an ultrasonic wave receiver 5. The output of the ultrasonic wave receiver 5 is inputted to a pulse width measuring circuit 9 through an amplifying circuit 6, a filter circuit 7, a waveform shaping circuit 8, and an AND gate 14 to measure the pulse width of the received signal. This pulse width is stored in a storage circuit 10. A comparing circuit 11 compares the measured value with the numeral stored in the storage circuit 10 and outputs a detection signal if the comparison difference is less than a certain value.