Non-volatile semiconductor memory device and method of rewriting data stored in non-volatile semiconductor memory device

There is provided a non-volatile semiconductor memory device, including (a) a first gate insulating film formed on a channel region of a semiconductor substrate, (b) a floating gate electrode formed on the first gate insulating film, (c) a second gate insulating film formed on the floating gate electrode, (d) a control gate electrode formed on the second gate insulating film, and (e) an electric power source applying a gradually increasing voltage across the control gate electrode and the semiconductor substrate, the electric power source varying both an increment by which the voltage is increased and a period of time during which the voltage is kept constant, while data is being rewritten. The non-volatile semiconductor memory device is capable of increasing a rate at which data stored therein is deleted, without deleting data more than necessary.