To provide an encryption communication system that can easily realize encryption data communication on an ATM network without the need for utilizing an encryption function between contractors or between users.

In the encryption communication system 10, a terminal 14 connected to a LAN 12 is connected to a terminal 18 connected to other LAN 16 via an ATM network 20, and an inter-network connection system is provided with IWU(Inter Working Unit) 22, 24 that convert/inversely convert LAN packets of the LAN into ATM cells of the ATM network or vice versa and add address information denoting a destination of the packets or cells to them at the same time and with NHS(Next Hop Server) that generate and manage the address information. Functions of reception/management of an encryption key and of encryption/decoding are added to the IWU 22, 24 and an encryption key generation and distribution function required for the encryption communication is added to the NHS 26, 28. In the case of conducting the encryption communication, the sender IWU 22 encrypts a plain text to generate an encryption message and transmits the encryption message to the destination IWU 24, and the IWU 24 decodes the encryption message to obtain the plain text.