Method of operating storage device

Disclosed is a method of operating a storage device including a NAND flash memory including memory cells grouped into blocks, each block being divided into pages. According to the method, a controller in the storage device loads, onto a memory region, a look-up table containing first read reference voltage sets corresponding to respective retention degradation stages of the NAND flash memory and second read reference voltages sets corresponding to respective pages which vary in terms of the threshold voltages. Subsequently, the controller performs a read operation on the memory cells on a per-block basis by using the first read reference voltage set corresponding to a current retention degradation stage, the second read reference voltage set corresponding to a current page, or both, until all of the memory cells in a current block are correctly read.