PURPOSE: To keep the gain variance of an equivalent transmitter within an optional range despite the fluctuation of number of working channels and to ensure an effective use of the electric power of the transmitter, by controlling with prescribed weighting the working point of the transmitter in response to the number of audio circuits.

CONSTITUTION: A voice 1 is supplied by a voice activator VOX to a delay circuit 11 which prevents the erasion of the head voice part and then modulated by a modulator 31. The modulated voice is applied to a voice detector 21. The number of working channels is varied with the VOX action, and the output of the voice detector of each channel is fetched into a counter 11 to calculate the number of working channels. This calculated number of working channels is delivered to a controller 12. The controller 12 controls a high-speed variable attenuator 13 and then the working point of a transmitter 9. Thus the transmitter is actuated at an optimum working point in response to the number of working channels, and the electric power of the transmitter can be used with the highest efficiency.