Patent US 5,574,686

An object is to realize a nonvolatile semiconductor storage system which can prevent a false reading operation due to the overerasure, improve the lower limit of operation margin, lower the supply voltage and form a signal power supply. When each of memory transistors 1-4 is subjected to the reading operation, a negative voltage is applied to a non-selected word line WL2 from X-decoder 5 and negative voltage generating circuit 8 to prevent the false reading operation due to the overerasure. When each of the memory transistors 1-4 is subjected to the erasing operations, a negative voltage is applied to word lines WL1 and WL2 to reduce a high voltage to be applied to a source line SL. This can realize low voltage operation and single voltage power supply operation. By applying the negative voltage to the substrate of a memory transistor when it is subjected to the reading operation, the false reading operation due to the overerasure can be prevented.