A non-volatile semiconductor memory device includes: a cell array having electrically rewritable and non-volatile memory cells arranged therein; and a sense amplifier circuit configured to detect voltage change of a bit line in the cell array, thereby reading data of a selected memory cell coupled to the bit line, wherein the sense amplifier circuit is controlled to read data at plural timings within a period in which the bit line voltage is changing in correspondence with the selected memory cell, and compare data read out by successive two data read operations with each other so as to judge a threshold margin of the selected memory cell.