Patent ID: 8358537

Claim:
A nonvolatile semiconductor memory device, comprising: a memory cell array including an array of NAND cell units, each of the NAND cell units further including a memory string and select transistors connected to both ends of the memory string, the memory string including a plurality of nonvolatile memory cells connected in series; word lines each connected to the nonvolatile memory cells; bit lines connected to first ends of the NAND cell units respectively; a source line connected to second ends of the NAND cell units; and a control circuit configured to select one of the nonvolatile memory cells as a selected memory cell to control a data reading operation on the selected memory cell, the nonvolatile memory cells being configured to have a plurality of threshold voltage distributions assigned to plural types of data to be stored therein, and the control circuit being configured to, when executing the data reading operation: apply to a selected word line, the selected word line being a word line connected to a selected memory cell, a first voltage obtained by adding a first adjusting voltage having a certain voltage value to an intermediate voltage between an upper limit value and a lower limit value of adjoining two of the threshold voltage distributions; apply to first non-selected word lines, the first non-selected word lines being word lines adjoining the selected word line, a second voltage obtained by subtracting a second adjusting voltage having a certain voltage value from a reading pass voltage for making the nonvolatile memory cells conductive regardless of a type of the plurality of threshold voltage distributions of the memory cells; apply to second non-selected word lines, the second non-selected word lines being word lines adjoining the first non-selected word lines, a third voltage obtained by adding the second adjusting voltage to the reading pass voltage; and apply to third non-selected word lines, the third non-selected word lines being non-selected word lines except the first non-selected word lines and the second non-selected word lines, the reading pass voltage.