Patent ID: 8649221

Claim:
A method of performing a reading operation in a nonvolatile semiconductor memory device, the nonvolatile semiconductor memory device including a memory cell array which comprises: a memory string having a plurality of memory cells connected in series therein; a first select transistor connected to one end of the memory string; a second select transistor connected to the other end of the memory string; a bit line connected to the memory string via the first select transistor; a source line connected to the memory string via the second select transistor; and a word line connected to a control gate electrode of each of the memory cells, the method of performing a reading operation being configured to apply a reading voltage to the control gate electrode of a selected memory cell in the memory string to determine whether the selected memory cell is conductive or not, and thereby execute data reading of the selected memory cell, the method comprising: performing a first reading operation configured to set a voltage between the control gate electrode and a source of the selected memory cell to a first value and thereby read a threshold voltage set in the selected memory cell; and performing a second reading operation configured to set the voltage between the control gate electrode and the source of the selected memory cell to a voltage value which is smaller than the first value, while keeping a voltage of the control gate electrode of the selected memory cell at 0 or a positive value, and thereby read again the threshold voltage set in the selected memory cell.