PURPOSE: To improve transmission efficiency by superposing a dial signal in the form of DC pulses directly upon a sound as it is and by transmitting it after ΔM modulation.

CONSTITUTION: Dial transmission input signal SS, after having a high frequency component of about 100Hz or above removed through low-pass filter 31, is inputted to ΔM coder 32 and sound transmission input 4WS is similarly inputted to coder 32 by way of band-pass filter 33. Coder 32 ΔM-encodes the sound signal and dial signal simultaneously and sends out ΔM output signal SOUT. Then, ΔM reception input Rin is decoded by ΔM decoder 34 and converted into an analog signal. Next, low- pass filter 35 extracts a dial signal component of about 100Hz and above to operate relay 37 and dial reception output SR is obtained via contact 38. Further, a 0.3∼ 3.4kHz component is selected by band-pass filter 39 to obtain sound reception output 4WR.