1. A multi-ROM writer which is selectively connected to various types of target equipment where each type of flash memory is mounted, and overwrites the flash memory mounted in the target equipment, comprising: a storage medium which stores an overwriting program corresponding to the flash memory of the target equipment and information to be written to the flash memory; reading means for reading the overwriting program and the information from the storage medium; and control means which operates using the overwriting program read by the reading means and overwrites the flash memory on the basis of the information read by the reading means, by controlling the transmission of data to the flash memory, the reception of data from the flash memory, and the output voltage switching which switches and outputs a voltage selected from a plurality of voltages.