An in-vehicle network system which is mounted on a vehicle includes an electronic control unit, a plurality of gateway devices, and a plurality of sensor devices that collects ambient information which is information around the vehicle. Each of the sensor devices communicates with the electronic control unit via at least one gateway device. The electronic control unit includes a mode management unit that determines one operation mode in a plurality of operation modes associated with the sensor device to be operated, and a sleep instruction control unit that specifies a gateway device which is the gateway device, in which the connected sensor device does not operate, and does not need to relay sensor information, based on the operation mode determined by the mode management unit, and transitions the gateway device having no need to relay the sensor information to a low power state in which a processing capacity is lowered.