PURPOSE: To send simultaneously a voice signal and a 2-channel data signal through one line by dividing a transmission frequency band of the line by a filter into 3 and connecting a voice signal input/output device to one band and connecting respectively a MODEM to the other two bands.

CONSTITUTION: A multiplex MODEM A has transformers 2, 3 and an adder 4 is connected to a transmission transformer 2, and an adder 4 adds a voice signal fed from an LPF 5 and a data signal fed from BPFs 9, 13 and gives the result to the transformer 2. The input signal is fed to an PLF 6 and BPFs 10, 14 from a transformer 3 and a telephone set 8 is connected to the LPFs 5, 6 via a telephone set interface (IF) section 7. A DTE 12 is connected to the BPFs 9, 10 via a MODEM 11. This is applied also to the DTE 16. Thus, the voice signal and the 2-channel data signal are sent simultaneously through one line.