An in-vehicle network system includes a power supply, an upper electronic control unit (ECU), an intermediate ECU configured to communicate with the upper ECU, and a plurality of lower ECUs configured to communicate with the intermediate ECU. The intermediate ECU is configured to receive power supplied from the power supply and supply the power supplied from the power supply to the lower ECUs. The lower ECUs are configured to transition, when the power is supplied from the intermediate ECU, from a power-off state to a standby state to wait for an instruction. The power supply is connected to the intermediate ECU via a power line and is not connected via the same power line to the upper ECU.