In a communication system including multiple ring networks, each ring network comprises: a host node serving as an upper controller for the other nodes in the own ring network, the host node being configured to be capable of communicating with another ring network; and a switch provided in the ring network and having multiple regular ports and multiple redundant ports, wherein the redundant communication ports of the switch in each ring network are connected to the redundant communication ports of the switch in the other ring network via redundant communication lines. When a communication error is detected in one ring network, the host node of the one ring network controls the switch of the own ring network and the switch of another ring network such that at least one node of the own ring network is incorporated in the other ring network via the switches and the redundant communication lines.