Abstract: In case of a trouble in any unit network of a communications network system which comprises a plurality of unit networks, communications are relieved and reliability of communications is improved. The respective unit networks are connected through internetwork connection circuits, and communications are normally made on each unit network by a communication path formed by performing loopback of remote communication devices at both ends of the individual unit network. If a trouble such as a disconnection fault occurred in a certain unit network, the loopback of the internetwork connection circuit of the remote communication devices at both ends of the unit network having the trouble is relieved to form an alternate communication path, and this alternate communication path is used to keep the communications between local communication terminals adjacent to the troubled spot.