PURPOSE: To provide a high resolution by transmitting a shooting sound in the multiple period of an acoustic energy having a profile having a clear peak of amplitude, and generating an echo having a maximum constant period.

CONSTITUTION: By the generation of a blanking signal in a port 6, the transmission sequence of a transmitter 22 is initialized to generate the burst of a high frequency electric energy applied to a transducer, and a distance counter 16 performs a counting operation. After the lapse of a blanking interval time, a receiver 24 receives the signal from the transmitter 22, and when the echo reflected from the surface of a target under monitoring is received and converted by the transducer 8, and recognized by the receiver 24, a counter stop is transmitted to the counter 16 to execute the interruption service routine to a processor 2. The processor 2 inputs the count from the counter 16 to confirm the echo. The peak amplitude of the echo waveform is attained by the timing of duration of energization of the transducer 8.