An electronic control unit connected to a bus of a first network where first-type frames are transmitted following a first communication protocol and a second network where second-type frames are transmitted following a second communication protocol in an onboard network system. The electronic control unit sequentially receives the first-type and the second-type frames from the bus and the second network respectively, and stores in first and second reception buffers, respectively. The electronic control unit sequentially generates first-type data and second-type data by referencing the contents of the first reception buffer and the second reception buffer, and stores the first-type and second-type data in first and second transmission buffers, respectively. The first-type data is traveling control data for a vehicle, and the second-type data is other data. The first-type or second-type data that is priority type data is transmitted with priority.