Non-volatile semiconductor memory device and rewriting method

A non-volatile semiconductor memory device, comprising:

• a memory array comprising a plurality of memory cells, wherein each of the plurality of memory cells is capable of storing a plurality of data values depending on the voltages thereof, the plurality of data values include a first data value corresponding to a first voltage range and a second data value corresponding to a second voltage range, and the first data is written in one memory cell of the plurality of memory cells;
• a determination section for determining whether a voltage value of the one memory cell is higher or lower than a reference value set between a maximum value and a minimum value of the first voltage range;
• and
• a rewrite section for rewriting the first data into the one memory cell based on a determination result of the determination section so that a margin between the first voltage range and the second voltage range in the one memory cell is enlarged.