A routing method and an in-vehicle gateway device having a plurality of interfaces for communication, automatic setting of routing function is enabled to perform, in consideration of characteristics of data which a connecting network handles. A network is classified to an information-system, a control-system and a safety-system, based on characteristics thereof, and which one of the systems includes a network connecting to the in-vehicle gateway device, is judged from equipments connecting to the network, or traffic of the network. Still more processing in data exchange among different classifications is set in advance.