Nonvolatile semiconductor memory device and method of operating the same

A nonvolatile semiconductor memory device, comprising:

• a cell array including a bit line, a plurality of word lines and a source line that intersect the bit line, and a cell string electrically connected between the bit line and the source line, the cell string including a plurality of memory cells connected to each other in series, each of the memory cells being configured from a transistor having a structure in which a charge accumulation layer and a control gate which is connected to one of the word lines are stacked on a semiconductor substrate via an insulating film;
• and
• a control unit that performs a data write/erase on the memory cell,
• the control unit, during the data erase, applying at least to the word line connected to the memory cell disposed most to a source line side a lower control voltage than that applied to the word line connected to the memory cell disposed most to a bit line side, of a plurality of the word lines connected to at least a plurality of the memory cells mutually written with data of an identical number of bits in the cell string.