ATM communication system capable of realizing a high speed and efficient datagram delivery for the connection-less communication among the terminals in the ATM network. In the system formed by a plurality of ATM networks inter-networking with each other, each network containing a plurality of terminals, the ATM networks with connection-less service function units for managing a connection-less datagram transmission are provided in the ATM networks, and the connection-less datagram transmission from each terminal to a destination terminal is performed by resolving a connection identifier for identifying an ATM connection connected to a destination side connection-less service function unit associated with a destination side ATM network containing the destination terminal, and transmitting datagram from said each terminal to the destination side connection-less service function unit through the ATM connection identified by the resolved connection identifier.