Abstract: A vehicle-mounted data rewriting control system includes a master control system. In rewriting data in electronic control systems, the master control unit first obtains through radio communication rewrite data from an external management center which holds and manages VIN codes of vehicles and version information of control programs. The supplied data are temporarily stored in a memory unit of the master control system, and the stored rewrite data are determined for their properness. By using the stored rewrite data, the data in the electronic control systems are rewritten on condition that the rewrite data stored in the memory unit are proper.