A vehicle onboard network system comprising: a vehicle onboard central control device configured to receive signals from a plurality of sensors mounted on a vehicle and generate a control signal for controlling onboard devices mounted on the vehicle; and a plurality of relay devices placed in respective zones in the vehicle and connected to the vehicle onboard central control device in a daisy chain manner via a backbone network MNW through which a digital signal of a predetermined protocol is transmitted, characterized in that the plurality of sensors being connected to the vehicle onboard central control device via some of the plurality of relay devices, the onboard devices being connected to the vehicle onboard central control device via some of the plurality of relay devices, wherein each of the relay devices includes a backbone-side communication port connected to the backbone network MNW, a plurality of device-side communication ports connected to onboard devices, and one or more first interface conversion devices (35, 61, 62, 63, 64) configured to perform interface conversion between the backbone-side communication port and the plurality of device-side communication ports, some of the plurality of device-side communication ports are a plurality of general-purpose communication ports to which an input circuit and/or output circuit of a type identical to a type of another relay device is connected, and one or more onboard devices which are capable of being directly driven by an ordinary output circuit or one or more onboard devices which are capable of being directly connected to an ordinary input circuit are directly connected to a part of the plurality of general-purpose communication ports, whereas one or more specific onboard devices having difficulty in driving with an ordinary output circuit or one or more specific onboard devices for which processing is difficult with an ordinary input circuit are connected to another part of the plurality of general-purpose communication ports via one or more second interface conversion devices, the one or more second interface conversion devices being configured to perform interface conversion. a vehicle onboard central control device configured to receive signals from a plurality of sensors mounted on a vehicle and generate a control signal for controlling onboard devices mounted on the vehicle; and a plurality of relay devices placed in respective zones in the vehicle and connected to the vehicle onboard central control device in a daisy chain manner via a backbone network MNW through which a digital signal of a predetermined protocol is transmitted, characterized in that the plurality of sensors being connected to the vehicle onboard central control device via some of the plurality of relay devices, the onboard devices being connected to the vehicle onboard central control device via some of the plurality of relay devices, wherein each of the relay devices includes a backbone-side communication port connected to the backbone network MNW, a plurality of device-side communication ports connected to onboard devices, and one or more first interface conversion devices (35, 61, 62, 63, 64) configured to perform interface conversion between the backbone-side communication port and the plurality of device-side communication ports, a backbone-side communication port connected to the backbone network MNW, a plurality of device-side communication ports connected to onboard devices, and one or more first interface conversion devices (35, 61, 62, 63, 64) configured to perform interface conversion between the backbone-side communication port and the plurality of device-side communication ports, some of the plurality of device-side communication ports are a plurality of general-purpose communication ports to which an input circuit and/or output circuit of a type identical to a type of another relay device is connected, and one or more onboard devices which are capable of being directly driven by an ordinary output circuit or one or more onboard devices which are capable of being directly connected to an ordinary input circuit are directly connected to a part of the plurality of general-purpose communication ports, whereas one or more specific onboard devices having difficulty in driving with an ordinary output circuit or one or more specific onboard devices for which processing is difficult with an ordinary input circuit are connected to another part of the plurality of general-purpose communication ports via one or more second interface conversion devices, the one or more second interface conversion devices being configured to perform interface conversion.