The invention aims to provide a nonvolatile semiconductor memory and a data writing method, which quickly and precisely accumulate a desired amount of charges corresponding to data-to-be-written in a charge accumulating part of a memory cell. When charges are injected into the charge accumulating part of the memory cell by applying a writing voltage corresponding to the data-to-be-written to the drain or source region of the memory cell, the writing voltage is reduced on the basis of an increase in the amount of charges accumulated in the charge accumulating part.