To surely continuously use specific data used by the old program prior to rewriting of an electronic control unit (ECU) even by a new program posterior to rewriting without limiting the program change of the ECU.

A program rewriting device connected to an ECU reads data used by the old program prior to rewriting of the ECU from a preservation requirement area in which data to be continuously used are stored regardless of the version of the program by the ECU before rewriting the program of the ECU, and saves the data to the memory of the device (S300). Then, when the rewriting of the program (S400) is ended, data to be used even by a new program posterior rewriting are selected from the saved data, and those respective data are rearranged so that the storage destination addresses in the preservation requirement area can be matched with a new program, and written in the preservation requirement area of the ECU (S500, S600).