PURPOSE: To detect exactly the distance from a forward obstruction without malfunction even with an unmanned car or the like to be used in a highly noisy factory by transmitting ultrasonic waves consisting of plural different frequency components toward the obstruction.

CONSTITUTION: A wave transmitting part 100 transmits the ultrasonic waves consisting of the plural frequency components from wave transmitters 133A, 133B toward the obstruction. Wave receiving parts 200A, 200B receive at least the ultrasonic waves of the respetive frequency components after the transmission from the part 100 and outputs the plural receiving timing signals 237A, 237B corresponding to the frequency components. the time data since the part 100 transmits the ultrasonic waves until the finally outputted reception signal is detected is made into a digital clock signal 413 by a data latch circuit 411. Said signal is conducted to a measuring circuit after digital-to-analog conversion 415 and the distance up to the obstruction is obtd.