A rewrite control apparatus capable of saving data which should not be reset, and rewriting a program of a large capacity on a memory of a small capacity. For this purpose, a data area (23b) for storing vehicle condition data is provided in a storage medium (23) of an onboard controller (35). When rewriting an onboard program, the apparatus writes the vehicle condition data into a storage area (23a) different from the data area, or a storage medium different from the storage medium (23) and save it, and executes the rewrite, and after finishing the rewrite, it writes the saved vehicle condition data back into the data area.