NAND flash memory device includes a common bit line, a first cell string including a first string selecting transistor having a first gate length, a second string selecting transistor having a second gate length, first cell transistors each having a third gate length and a first ground selecting transistor having a fourth gate length, a second cell string including a third string selecting transistor having the first gate length, a fourth string selecting transistor having the second gate length, second cell transistors each having the third gate length and a second ground selecting transistor having the fourth gate length and a common source line commonly connected to end portions of the first and second ground selecting transistors included in the first and second cell strings. At least one of the first gate length and the second gate length is smaller than the fourth gate length.