Device for interconnection (2.1) of at least two data communication networks, connecting a first network referred to as the high security network and at least one second network referred to as the low security network, comprising: - a first interface for communication (2.11) with the high security network; - a second interface for communication (2.12) with the low security network; - a routing module (2.3) connected to the first interface; - an adaptation module (2.8) connected to the second interface; characterized in that it furthermore comprises: - a monodirectional path (2.4, 2.6), termed the downpath, between the routing module (2.3) and the adaptation module (2.8), allowing the transfer of data from the routing module (2.3) destined for the adaptation module (2.8), the monodirectional path comprising a diode prohibiting any uploading of data through the downpath; and - a monodirectional path (2.5, 2.7, 2.8), termed the return path, between the adaptation module (2.8) and the routing module (2.3), allowing the transfer of data from the adaptation module (2.8) destined for the routing module (2.3), the return path comprising a path management module and a firewall placed between the path management module and the adaptation module, and the adaptation module (2.8) forming the link between the monodirectional paths and the second interface (2.12) and formats the data transmitted on the return path according to a protocol different from that used on the high and low security networks, the path management module reconstructing the data transmitted on the return path on the basis of the protocol used by the adaptation module to format the data transmitted, the firewall filtering the data transmitted according to the protocol in such a way as to guarantee that any datum travelling through the return path is compliant with the said protocol. - a first interface for communication (2.11) with the high security network; - a second interface for communication (2.12) with the low security network; - a routing module (2.3) connected to the first interface; - an adaptation module (2.8) connected to the second interface; - a monodirectional path (2.4, 2.6), termed the downpath, between the routing module (2.3) and the adaptation module (2.8), allowing the transfer of data from the routing module (2.3) destined for the adaptation module (2.8), the monodirectional path comprising a diode prohibiting any uploading of data through the downpath; and - a monodirectional path (2.5, 2.7, 2.8), termed the return path, between the adaptation module (2.8) and the routing module (2.3), allowing the transfer of data from the adaptation module (2.8) destined for the routing module (2.3), the return path comprising a path management module and a firewall placed between the path management module and the adaptation module, and the adaptation module (2.8) forming the link between the monodirectional paths and the second interface (2.12) and formats the data transmitted on the return path according to a protocol different from that used on the high and low security networks, the path management module reconstructing the data transmitted on the return path on the basis of the protocol used by the adaptation module to format the data transmitted, the firewall filtering the data transmitted according to the protocol in such a way as to guarantee that any datum travelling through the return path is compliant with the said protocol.