A double ring network system configured with two or more transmission stations each including a pair of bi-directionally communicative communication ports, including any adjacent two transmission stations as terminal stations, and any transmission station as a base, as mutually connected in a ring form by communication ports through a transmission line, for intercommunications between transmission stations, in which the transmission stations are each adapted to determine whether or not frame signals are received from adjacent transmission stations, transmit route identifying frame signals for identification of routes to adjacent transmission stations, when having failed in reception, and set own station as a terminal station substituting for a current terminal station, when having received no responses to the route identifying frames from adjacent transmission stations, so they are kept from arriving at ring-like connected subsequent transmission stations.