PURPOSE: To enable efficient and highly measurement of distance, by gauging relative distance between the target and a sonar based on the difference in the dependence on the frequency between a sound emission source spectrum and an audible sound spectrum.

CONSTITUTION: An input sound wave 101 emitted from a target sound emission source is received with a receiver 1 and converted into an electrical signal to be fed to a receiver 2, the output of which is fed to a recording display circuit 1000 and an audible spectrum calculator 3. The calculator 3 calculates the difference due to frequency from an audible spectrum computed to be fed to a distance measuring device 4. A sound emission source spectrum generator 5 memorizes a sound emission source spectrum and data pertaining the coefficient of absorption loss over a specified frequency band beforehand and supplies the data to the measuring device 4. Then the measuring device 4 executes computation by a specified formula and outputs the results to the line 401 as measured distance. This facilitates the measurement of distance based on the difference between the sound emission source spectrum and the audible sound spectrum only through a wave receiving function.