PURPOSE: To reduce the hardware quantity by synthesizing the conference call signal with an analog signal and transmitting them after converting them into digital signals.

CONSTITUTION: The call signal sent from a general telephone 10 is converted into a digital signal by a digital line circuit 20 and supplied to an analog 3-person call trunk 50 via a digital network 30. Thus the digital signal is converted into an analog signal by a decoder 101 and supplied to an analog synthesizing part 2 via a hybrid 102 and then to hybrids 202 and 302 via capacitors C1∼C3. The 4-wire transmission side outputs of both hybrids 202 and 302 are converted into digital forms by coders 200 and 300 and sent to digital line circuits 21 and 22 via the network 30. The circuits 21 and 22 convert the digital signals onto analog signals and transmit them simultaneously to general telephones 11 and 12. Thus the talking is possible among three persons.