To provide a packet communication system which can recognize the connection information of an X.25-IP protocol converter connected newly to a network by an existing X.25-IP protocol converter.

When a terminal side communication unit 13 receives a call connection request from an X.25 packet terminal 3, the X.25-IP protocol converter 1 transmits a DTE address, a user ID, and a password as an IP packet from a converter side communication unit 12 toward the X.25-IP protocol converter 2. When the converter side communication unit 23 receives an IP packet from the X.25-IP protocol converter 1 by the converter side communication unit 23, the X.25-IP protocol converter 2 stores the authentication of the connection and the DTE address and the IP address to a DTE address-IP address conversion table 21 based on the content of the IP packet at a connection authentication storage 22.