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 by the first-type frame including 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 which includes information obtained from a device connected (300a; 300b; 300c) to the electronic control unit; 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 (200b - 200c) 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; characterized by the first-type frame including 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 which includes information obtained from a device connected (300a; 300b; 300c) to the electronic control unit; 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 (200b - 200c) 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. 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.