The invention provides a nonvolatile semiconductor storage apparatus capable of inhibiting reliability degradation caused by data rewriting, and a data erasure method therefor. An erasure method for a flash memory comprises the steps of keeping a voltage of a control gate at a value of 0V; applying a high-voltage erasure pulse (Ps) to a P well (14); keeping the voltage of the control gate at the value of 0V after enabling electrons to be released to the P well (14) by a floating gate; and applying a weak erasure pulse (Pw) with a voltage lower than that of the erasure pulse (Ps) to the P well (14).