To communicate through an IP network with an inexpensive constitution in a network communication system using a specific protocol like X.25.

Gateway devices and address resoluting servers are provided on the IP network by areas; and when receiving a call request packet from a packet terminal (a) using X.25, a gateway device XGW-A acquires IP addresses corresponding to the calling address and the called address of the call request packet from an address resolution server A installed in the same area and performs connection processing between the packet terminal (a) and a packet terminal (e) of the destination. Thus each packet terminal is stored on the IP network and can communicate though the IP address of the packet terminal is not held on the gateway device side.