PURPOSE: To detect a target echo by a small-scale circuit by detecting an input signal exceeding a certain threshold level continuously for longer than a certain time.

CONSTITUTION: An envelope detecting circuit 1 detects the envelope of an input signal (a) and a comparator 2 compares the detection output (b) with the certain threshold level and generates '1' as its comparator output (c) when the detection output is larger than the threshold level. A timer 3 starts counting down from a reference time T with rising pulses of the output (c). When the timer 3 clocks the time T, the output (d) of a counter 0 becomes '1' and the echo detection output (e) of an AND gate 4 becomes '1', so that the echo is detected. If the output (c) becomes '0' before the timer 3 clocks the time T, the timer 3 stops and the output (e) is still '0', so that no echo is detected.