Abstract: A vehicle-onboard control system for controlling various devices mounted on the motor vehicle. A control program for the control system can be altered through facilitated processing procedure with simple hardware structure while avoiding unfitness of control variables stored in a backup random access memory and used in executing the control program. A bootstrap program (13) responds to a command signal (C) issued by an external memory rewriting device (50) upon rewriting of the control program to thereby set an initialization flag (FN) in a backup RAM (14) containing control variables. The control program rewritten with updated data supplied from the memory rewriting device (50) responds to the set state of the initialization flag (FN) upon activation, to thereby initialize the control variables stored in the backup RAM (14) so that the control variables conform with the control program.