An electronic control unit (ECU) is provided. The electronic control unit is connected to a first network in an onboard network system. The onboard network system includes the first network and a second network. In the first network, first-type frames are transmitted following a first communication protocol. In the second network, second-type frames are transmitted following a second communication protocol. The electronic control unit generates first-type frames following the first communication protocol, and transmits the generated first-type frames to the first network. The first-type frame includes first information and second information. The first information serves as a base for a second-type frame to be transmitted to the second network. The second information indicates information that the first-type frame is to be transmitted to the second network.