A communications network system which is capable of continuously performing a normal communications of data on a communication cable. A Navigation unit 30 transmits an intersection data having ID0001 to an automatic transmission control unit 10 via communications cable. A gateway 50 which accepts communications from an information system network B to a control system network A is intervened on a communications line between both networks. When detecting a sending data having an ID code of ID0001, gateway 50 compares its communication period 0 of the data with previously stored communications period. If they are coincident with each other, the communications from network B to network A is enabled. If communications periods are incoincident with each other, the ID code when the intersection data is transmitted is changed to ID0110 and communications enabling (accepting) ID code by gateway is also changed from ID0001 to ID0110.