A program method includes applying a first voltage to a plurality of bit lines, applying a second voltage to a common source line (CSL), and performing a program loop by applying a program voltage and a verify voltage to each of a plurality of ground selection lines (GSLs) positioned between one bit line among the plurality of bit lines and the CSL. The program loop is performed on both a program completed cell in which a program is completed by applying the program voltage and a program target cell.