In a 3Tr. NAND including a cell unit constituted of one memory cell and
two select gate transistors between which the cell is held, to renew data
by a byte unit, at an erase time, a potential of a bit line or source
line can be set by the byte unit, so that erase by the byte unit is
possible. Accordingly, with respect to only the data of the memory cell
which is a renewal object, an erase/write operation is performed, and
reliability of a memory operation is enhanced.