A magnetic field corresponding to an acoustic signal is generated from an antenna, and by using a coil, can be again converted to an acoustic signal. It is possible to estimate where the invisible antenna is with the distribution of the received signal. The estimation is applied to a maintenance of a gas pipe on the situation that the distance from the entrance to a maintenance area is known, but piping route isn’t. It is possible to identify maintenance areas of a gas pipe by inserting the antenna to it. The estimation has been done by listening to the received signal manually. However, it is difficult for people to identify accurate point because the difference in the volume for each places is subtle. To solve this problem, we visualized the distribution of the received signal, and estimated the magnetic field with only the acoustic signal. Then, we proposed a method to calculate where the invisible antenna is automatically by using symmetry of the distribution of the received signal.