A duel program verify operation is performed using first and second verify voltages. In order to reduce the width of a threshold voltage distribution during an incremental step pulse program implementation, data of a corresponding memory cell are verified twice using the first verify voltage and the second verify voltage. During a second verify operation using the second verify voltage, a sensing current is adjusted by controlling voltages applied as a bit line select signal and an evaluation time period. Therefore, the threshold voltage of the memory cell can be measured higher or lower than its actual value and the width of a threshold voltage distribution is reduced.