A memory system including a nonvolatile semiconductor memory device and a controller. The memory device includes a plurality of word lines; and a plurality of memory cells each connected to a corresponding one of the word lines and each having N threshold voltages for storing multi-valued level, where N is a natural number of 4 or greater; wherein stored data in each of the plurality of memory cells constitutes a plurality of pages, at least only a part of multi-value level is used for storing data, a data "1" is always written to a lower page, a "0" or "1" binary data is written to an upper page, the same data is written in each of the pages when writing in the nonvolatile memory device, and only part of the pages to which the same data is written is accessed when reading out the nonvolatile memory device.