PURPOSE: To facilitate the discrimination between a signal and noise and to enhance the discrimination capacity of a target, by uniformizing a reflected signal different in a peak level to display the same.

CONSTITUTION: The sonic wave from a transmitter is reflected from a target to be received by a receiver 1 and converted to an electric signal. Subsequently, said electric signal is converted to a digital receiving signal by an A/D converter circuit 2 to be sent to the first operation circuit 3 and converted to a positive envelope detection signal. This signal V is sent to the first memory circuit 5 to be stored therein and, at the same time, also sent to an average value circuit 4. The circuit 4 calculates the average level -V of a definite period containing the reflected signal in the signal sent from the circuit 3 to send the same to the second memory circuit 6 and calculates the average level -N only of the noise of the definite period to send the same to the third memory circuit 7. These data stored in three memory circuits are sent to the second operation circuit 8 to perform operations shown by formulae I, II, III. The calculated sample data of the formula III is displayed on a display circuit 9 to make it possible to uniformize a reflected signal level.