An electronic control unit connected to a first network in an onboard network system, the onboard network system including the first network for transmission of a first-type frame following a first communication protocol, and to a second network for transmission of a second-type frame following a second communication protocol that is different from the first communication protocol, the electronic control unit comprising: a generating unit adapted to generate the first-type frame following the first communication protocol; and a transmission unit adapted to transmit the first-type frame generated by the generating unit to the first network; characterized in that the first-type frame includes first information serving as a base for the second-type frame to be transmitted to the second network, and second information indicating that the first-type frame includes information that is to be transmitted to the second network, and a reception unit adapted to receive external information indicating information on a state of a device on the onboard network system which is received from another electronic control unit connected to the first network or the second network, or external information indicating information which is received from a communication module configured to communicate with a server via an external network; wherein the generating unit is adapted to generate, based on the received external information, one or more types of information to be transmitted to another electronic control unit and to determine whether a transmission destination for a generated type of information is an electronic control unit connectable to the second network or not in accordance with the generated type of the information, wherein the generating unit: generates, in a first case where the transmission destination for said generated type of information is an electronic control unit connectable to the second network, the first-type frame including the first information generated based on the external information and including the second information; and generates, in a second case where the transmission destination for said generated type of information is not an electronic control unit connectable to the second network, the first-type frame including information generated based on the external information and including third information that is contrary to the second information. a generating unit adapted to generate the first-type frame following the first communication protocol; and a transmission unit adapted to transmit the first-type frame generated by the generating unit to the first network; external information indicating information on a state of a device on the onboard network system which is received from another electronic control unit connected to the first network or the second network, or external information indicating information which is received from a communication module configured to communicate with a server via an external network; generates, in a first case where the transmission destination for said generated type of information is an electronic control unit connectable to the second network, the first-type frame including the first information generated based on the external information and including the second information; and generates, in a second case where the transmission destination for said generated type of information is not an electronic control unit connectable to the second network, the first-type frame including information generated based on the external information and including third information that is contrary to the second information.