A frame generating method of generating a frame to be transmitted, by 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 including a second network for transmission of a second-type frame following a second communication protocol that is different from the first communication protocol, the method comprising:
- generating, by the electronic control unit, the first-type frame following the first communication protocol;
- and
- transmitting, by the electronic control unit, the first-type frame to the first network,
- wherein 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.