1. A method of driving a flash memory device, the method comprising: performing an erasing operation on memory cells; after the performing the erasing operation, performing a post-programming operation to control a threshold voltage of the memory cells; and after the performing the post-programming operation, performing a main programming operation on the memory cells, wherein the performing of the post-programming operation comprises increasing the threshold voltage of the memory cells in an erased state, thereby reducing a difference in the threshold voltage between the memory cells in the erased state and the memory cells in the programmed state, and the threshold voltage of at least one of the memory cells on which the post-programming operation has been performed is 0V or greater.