A specific on-board device, that is any one of a plurality of on-board devices, is configured to acquire the versions of software included in other on-board devices when the power source of a vehicle is turned on. The specific on-board device determines whether or not the versions of the software of all the on-board devices that constitute a vehicle control system are consistent, by comparing the version of the software of the specific on-board device and the versions of the software acquired from the other on-board devices with first consistency information prestored.