An apparatus for securely protecting data in a flash memory upon power off is disclosed. In the apparatus, a power detector monitors a voltage output from a power supply unit, and outputs a power fail signal when the voltage drops by a predetermined reference voltage or more. A Programmable Logic Device (PLD) outputs a Write Protect (WP) signal for performing write protection on the flash memory upon receiving the power fail signal from the power detector. A WP controller outputs the WP signal output from the PLD to the flash memory, according to a Ready/Busy (R/B) state of the flash memory.