An electronic control unit (ECU) is provided. The ECU 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 ECU generates first-type frames following the first communication protocol, and transmits the generated first-type frames to the first network. The ECU receives external information indicating state information of a device on the onboard network system received from another electronic control unit connected to the first network or the second network, or receives external information indicating information received from a communication module configured to communicate with the server via an external network.