A nonvolatile semiconductor storage device includes a plurality of cells for storing data on a basis of charges stored nonvolatilly, a write unit for writing and erasing data on the cell by injecting or extracting charges into or from the cell, a comparator for comparing the voltage produced by a selected cell to be read out with a threshold, a read unit for outputting read data on the basis of the comparison result by the comparator, and a threshold update unit for updating the threshold of the comparator according to the voltage produced by the selected cell.