To provide a memory-writing device which can simply and reliably write desired data to a nonvolatile memory of an electronic device, connection is made with an ECU to perform write processing to write write data from the memory-writing device to a flash ROM by copying a write-control program from the external portion to a RAM and executing the write-control program, and by sequentially sending the foregoing write-control program and write data to this ECU together with sending, at a predetermined timing, write-control information required for the ECU to execute the write-control program, the write-control program and the write-control information are stored in a freely attachable and removable first ROM and the write-control information stored in the first ROM is read and sent to the ECU at a memory-rewriting device to cause the write processing to be performed in the ECU. According to this device, an ECU of differing specifications can be supported merely by exchanging the first ROM.