An in-vehicle network system comprising: a power supply; an upper electronic control unit; a plurality of intermediate electronic control units (21, 22); and a plurality of lower electronic control units (31, 32) subordinate to each of the intermediate electronic control units, wherein: an intermediate electronic control unit (21, 22) is configured to: receive power supplied from the power supply; maintain the lower electronic control units subordinate to the intermediate electronic control unit in a power-off state until the intermediate electronic control unit receives a message from the upper electronic control unit; communicate with the upper electronic control unit and function as a gateway configured to relay communication between the upper electronic control unit and the plurality of lower electronic control units subordinate to the intermediate electronic control unit; transition from a standby state to a startup state and supply the power supplied from the power supply to the lower electronic control units subordinate to the intermediate electronic control unit in response to the message transmitted from the upper electronic control unit; and transmit a message to the lower electronic control units subordinate to the intermediate electronic control unit in the standby state in response to the message transmitted from the upper electronic control unit, and: the lower electronic control units (31, 32) subordinate to the intermediate electronic control unit are configured to: transition, when the power is supplied from the power supply, from the power-off state to a standby state to wait for an instruction; communicate with the intermediate electronic control unit; transition from the standby state to a startup state in response to the message transmitted from the intermediate electronic control unit; and specify an operation in the startup state in response to the message transmitted from the intermediate electronic control unit. a power supply; an upper electronic control unit; a plurality of intermediate electronic control units (21, 22); and a plurality of lower electronic control units (31, 32) subordinate to each of the intermediate electronic control units, receive power supplied from the power supply; maintain the lower electronic control units subordinate to the intermediate electronic control unit in a power-off state until the intermediate electronic control unit receives a message from the upper electronic control unit; communicate with the upper electronic control unit and function as a gateway configured to relay communication between the upper electronic control unit and the plurality of lower electronic control units subordinate to the intermediate electronic control unit; transition from a standby state to a startup state and supply the power supplied from the power supply to the lower electronic control units subordinate to the intermediate electronic control unit in response to the message transmitted from the upper electronic control unit; and transmit a message to the lower electronic control units subordinate to the intermediate electronic control unit in the standby state in response to the message transmitted from the upper electronic control unit, and: the lower electronic control units (31, 32) subordinate to the intermediate electronic control unit are configured to: transition, when the power is supplied from the power supply, from the power-off state to a standby state to wait for an instruction; communicate with the intermediate electronic control unit; transition from the standby state to a startup state in response to the message transmitted from the intermediate electronic control unit; and specify an operation in the startup state in response to the message transmitted from the intermediate electronic control unit. transition, when the power is supplied from the power supply, from the power-off state to a standby state to wait for an instruction; communicate with the intermediate electronic control unit; transition from the standby state to a startup state in response to the message transmitted from the intermediate electronic control unit; and specify an operation in the startup state in response to the message transmitted from the intermediate electronic control unit.