PURPOSE: To provide a nonvolatile semiconductor memory device, capable of making automatic reading of data stored in a memory circuit impossible after a given period of time, and reading method thereof.

CONSTITUTION: This memory device is constituted of an opposite conductivity type source area and drain area, formed so as to be separated from each other on one conductivity type semiconductor substrate, and a gate unit, formed sequentially of a plurality of insulating films and the conductive film of uppermost layers on a channel area between the source area and the drain area while a given voltage is supplied from a first constant voltage source 15 to the gate terminal 13 of a semiconductor nonvolatile memory transistor 11 having an order of catching a charge into at least one of a plurality of insulating films. Further, a given voltage is supplied from a second constant voltage source 21 to a drain terminal 17 through a resistor element 18 and the voltage of the drain terminal 17 is compared with the output voltage of a voltage generating circuit 23, capable of setting the output voltage arbitrarily, by a comparator 27.