Nonvolatile semiconductor memory device and method of operating the same

A method of operating a nonvolatile semiconductor memory device, the nonvolatile semiconductor memory device comprising a memory cell array having a plurality of electrically rewritable memory transistors arranged therein and a control unit configured to govern control of an operation to the memory transistor, in order to erase data of a selected memory transistor, the method comprising:

• repeating a voltage application operation and a step-down operation, the voltage application operation applying an erase voltage to a selected memory transistor to change a threshold voltage at which the selected memory transistor is conductive, and the step-down operation, in the case where a threshold voltage of the selected memory transistor has not changed to a desired value, decrease the erase voltage by an amount of a certain step-down value;
• and
• controlling the step-down operation to monotonically decrease the step-down value so as to reduce the value of the erase voltage close to zero as the number of times of the voltage application operations increases.