An electronic control unit connected to a first network and to a second network in an onboard network system that includes the first network where transmission of first-type frames relating to traveling control of a vehicle is performed over a network, and the second network where transmission of second-type frames is performed, the electronic control unit comprising: a first reception buffer; a second reception buffer; a first transmission buffer; a second transmission buffer; a first reception unit that sequentially receives the first-type frames from the first network and stores data within the first-type frames in the first reception buffer; a second reception unit that sequentially receives the second-type frames from the second network and stores data within the second-type frames in the second reception buffer; characterized by further comprising: a generating unit that sequentially generates first-type data that is data for traveling control of the vehicle, and second-type data that is data used for other than traveling control of the vehicle, by referencing the contents of the first reception buffer and the second reception buffer, stores the generated first-type data in the first transmission buffer, and stores the generated second-type data in the second transmission buffer; and a transmission unit that transmits yet-to-be-transmitted first-type data in the first transmission buffer and yet-to-be-transmitted second-type data in the second transmission buffer, wherein the transmission unit performs priority transmission control, where priority type data, which is one of the first-type data and second-type data, is transmitted with priority over non-priority type data that is another type than the first-type data and second-type data. a first reception buffer; a second reception buffer; a first transmission buffer; a second transmission buffer; a first reception unit that sequentially receives the first-type frames from the first network and stores data within the first-type frames in the first reception buffer; a second reception unit that sequentially receives the second-type frames from the second network and stores data within the second-type frames in the second reception buffer; characterized by further comprising: a generating unit that sequentially generates first-type data that is data for traveling control of the vehicle, and second-type data that is data used for other than traveling control of the vehicle, by referencing the contents of the first reception buffer and the second reception buffer, stores the generated first-type data in the first transmission buffer, and stores the generated second-type data in the second transmission buffer; and a transmission unit that transmits yet-to-be-transmitted first-type data in the first transmission buffer and yet-to-be-transmitted second-type data in the second transmission buffer, wherein the transmission unit performs priority transmission control, where priority type data, which is one of the first-type data and second-type data, is transmitted with priority over non-priority type data that is another type than the first-type data and second-type data. a generating unit that sequentially generates first-type data that is data for traveling control of the vehicle, and second-type data that is data used for other than traveling control of the vehicle, by referencing the contents of the first reception buffer and the second reception buffer, stores the generated first-type data in the first transmission buffer, and stores the generated second-type data in the second transmission buffer; and a transmission unit that transmits yet-to-be-transmitted first-type data in the first transmission buffer and yet-to-be-transmitted second-type data in the second transmission buffer, wherein the transmission unit performs priority transmission control, where priority type data, which is one of the first-type data and second-type data, is transmitted with priority over non-priority type data that is another type than the first-type data and second-type data.