A memory device in which a write error can be prevented is provided. The memory device includes a NAND cell unit including a plurality of memory cells connected in series, a first selection transistor connected to one of terminals of the NAND cell unit, a second selection transistor connected to the other of the terminals of the NAND cell unit, a source line connected to the first selection transistor, and a bit line which intersects with the source line and is connected to the second selection transistor. In the memory device, a channel region of each of the first selection transistor and the second selection transistor is formed in an oxide semiconductor layer.