In a data write apparatus to a flash electrically erasable programmable read only memory (EEPROM) built in a microcomputer which is mounted on a circuit board, a write control section first initializes the flash EEPROM to allow data to be written in the flash EEPROM, and supplies a signal indicative of the data for the flash EEPROM. A level converting section convertes a level of the data signal such that the data signal level matches to an actual operation voltage level of the flash EEPROM and supplies the converted data signal to the flash EEPROM such that the data is written in the flash EEPROM.