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.