To suppress deterioration of the voice quality even when the communication of voice data is performed via plural data networks, by preparing a coding system selection control means among the data networks to secure the negotiation with other devices.

A terminal 10 notifies a voice coding/decoding device 12 of a 1st stage of the type of an available voice coding system in a call selection mode. Receiving the information on the voice coding system, the device 12 decides the next data network to be connected and also a coding/decoding device 13 to have negotiation with the data network based on the path information on a communicating destination IP address, etc. A coding system that has the least or desirably zero execution frequency or coding/decoding operations is selected based on the type of coding system notified by the terminal 10. Then the voice coding/decoding devices which are connected among the data networks are set by the selected coding system.