A communication system comprising: three or more nodes , and a multi-core fiber having a plurality of cores, the multi-core fiber being used in at least a partial segment of a connection between the nodes, characterized in that it further comprises: a fault information transmitting device , provided in each of the nodes, configured to transmit fault information indicating that a fault has occurred in a communication path between own node and another node of the nodes and the fault has not occurred in a communication path between the own node and another node of the nodes; and a fault location specifying device configured to specify a section between nodes in which the fault has occurred on the basis of a combination of the communication path in which the fault has not occurred and the communication path in which the fault has occurred indicated by the fault information received from the fault information transmitting device provided in each of the nodes, using a combination of the presence or absence of the fault in respective communication paths formed between the nodes and a predetermined logical formula based on the connection between the nodes and the respective communication paths. three or more nodes , and a multi-core fiber having a plurality of cores, the multi-core fiber being used in at least a partial segment of a connection between the nodes, characterized in that it further comprises: a fault information transmitting device , provided in each of the nodes, configured to transmit fault information indicating that a fault has occurred in a communication path between own node and another node of the nodes and the fault has not occurred in a communication path between the own node and another node of the nodes; and a fault location specifying device configured to specify a section between nodes in which the fault has occurred on the basis of a combination of the communication path in which the fault has not occurred and the communication path in which the fault has occurred indicated by the fault information received from the fault information transmitting device provided in each of the nodes, using a combination of the presence or absence of the fault in respective communication paths formed between the nodes and a predetermined logical formula based on the connection between the nodes and the respective communication paths. a fault information transmitting device , provided in each of the nodes, configured to transmit fault information indicating that a fault has occurred in a communication path between own node and another node of the nodes and the fault has not occurred in a communication path between the own node and another node of the nodes; and a fault location specifying device configured to specify a section between nodes in which the fault has occurred on the basis of a combination of the communication path in which the fault has not occurred and the communication path in which the fault has occurred indicated by the fault information received from the fault information transmitting device provided in each of the nodes, using a combination of the presence or absence of the fault in respective communication paths formed between the nodes and a predetermined logical formula based on the connection between the nodes and the respective communication paths. <EOS>