PURPOSE: To prevent howling due to acoustic coupling, by stopping the function of a microphone in case the distance between the microphone and a speaker is decreased less than a prescribed distance.

CONSTITUTION: If a microphone 13 is set at a place shorter than a prescribed distance from a speaker box 5, an induced electromagnetic field generated from an induced electromagnetic field oscillator 6 is received at a receiver within a microphone case 9. Then a detecting signal is delivered. A control circuit 11 works with the detecting signal, and a relay 12 works to open a contact with contacts 15 and 16 closed. Therefore the microphone 13 is cut off from a loudspeaker circuit 8, thus preventing the howling due to the acoustic coupling when the distance between the microphone 13 and a speaker 7 is reduced less than a prescribed distance.