PURPOSE: To control automatically the mixing ratio between the radio signal and the hearing microphone signal in response to the ambient condition, by changing said mixing ratio in response to the level of the microphone signal.

CONSTITUTION: A control voltage generating circuit 22 generates the voltage signal corresponding at least to the level of a sound band component among microphone signals and then applies the voltage signal to control circuits 8 and 18 in the form of the control voltage which decides the mixing ratio of a mixing circuit 24. The circuit 8 varies the level of the radio signal in response to the level of the control voltage applied from the circuit 22 and then applies the radio signal to an input terminal of the circuit 24. The circut 18 varies the level of the microphone signal in response to the level of the control voltage applied from the circuit 22 and applies the microphone signal to the other input terminal of the circuit 24. The circuit 24 mixes the audio signals sent from the circuits 8 and 18 and synthesizes them into an audio signal.