PURPOSE: To prevent the deterioration of a sound signal by using a frequency band compressing device which compresses the frequency band of the sound signal and sends it to a circuit and a frequency expanding device which expands the frequency band of a received sound signal and delivers it to a circuit respectively.

CONSTITUTION: The signals transmitted through a low pass filter LPF 12 and a band pass filter BPF 13 are supplied to a frequency band expander 10. The signals passed through a BPF 14 are supplied to a MODEM circuit 19. An LPF 15 and PBF 16 and 17 are connected to a circuit for transmission and allocate the prescribed signals to each frequency band to deliver them to the circuit for transmission. The sound signals sent from a frequency band compressor 11 are supplied to the LPF 15 and the BPF 16. While the data signal sent from the circuit 19 is supplied to the BPF 17. The circuit 19 is connected to a data terminal equipment DTE 3 and modulates the data signal into the signal that can be transmitted to deliver it to the BPF 17. At the same time, the circuit 19 demodulates the signal sent from the BPF 14 and delivers it to the DTE 3. This prevents the deterioration of the sound quality.