PURPOSE: To prevent the adverse influence of a multiply transmitted voice signal on the modulated signal of digital data, and to prevent the deterioration of the S/N of the received voice signal by limiting the voice signal level of the multiply transmitted voice signal within a fixed range.

CONSTITUTION: The voice signal transmitted from a microphone 1 and an amplifier 2 is inputted to a voice level limiting circuit 21. When the level of the inputted voice signal is small, the amplification factor of the voice level limiting circuit 21 is fixed to the maximum value, so that the output of the voice level limiting circuit 21 can be amplified by the maximum amplification factor. And also, when the level of the inputted voice signal is large, the amplification factor of the voice level limiting circuit 21 is decreased almost in inverse proportion to the size of the inputted voice signal. Thus, the change of the level of the voice signal outputted from the voice level limiting circuit 21 can be extremely reduced, and the output of the voice level limiting circuit 21 is applied through a high-pass filter 3 and a low-pass filter 4 to an adder 5.