PURPOSE: To improve S/N over a wide band by applying 1/2 frequency division to an output AC signal of a modulation circuit of a speaker inputting an acoustic signal and driving the speaker with a call signal superimposed with a DC component to reduce the distortion of an output signal from a carbon microphone.

CONSTITUTION: A transmission section 14 consists of a modulation circuit 2, a frequency division circuit 3 converting the modulated AC signal into a half frequency, a low-pass filter 4, an amplifier 5, a DC superimposing circuit 6 superimposing a DC on the AC signal, a speaker 7 inputting the acoustic signal to a telephone set transmission microphone and a control section 8 adjusting the timing of data transmission. A DC voltage in matching with the condition of Reo/δ=xAC.xDC is superimposed in the AC signal being the result of frequency division to an output signal of the modulation circuit 2 in the DC superimposing circuit 6 and the speaker 7 is driven, then the output current of the telephone set transmission carbon microphone does not include the 2nd harmonic having the frequency equal to the carrier to reduce the deterioration in S/N in the conversion system of the carbon microphone.