In a control information rewriting system, a rewriting device for vehicle ECUs and a control center are provided. A function f necessary for the rewriting device to access the ECUs is stored the control center. When data communication is carried out by way of a telephone line network, the control center acquires ID information from the rewriting device and acquires a telephone number of the call origin. When the association relationship between this ID and telephone number matches an association relationship stored in the database, the control center determines the rewriting device to be a legitimate one. Only when the control center determines that the rewriting device is a legitimate one, it transmits the function f to the rewriting device to enable rewriting of control information of the ECUs.