According to one embodiment, a storage device comprises a nonvolatile memory and a memory controller that performs reading and writing data from and into the nonvolatile memory. A number, not being 2n, of threshold areas can be set in the memory cells of the nonvolatile memory. The memory controller performs first writing based on first data value assignment, which sets 2n data values to correspond to 2n threshold areas, in first-time writing into a first memory cell of the nonvolatile memory and performs second writing on the first memory cell after the first writing without erasing data based on second data value assignment, which sets 2n data values to correspond to 2n threshold areas including threshold areas not used in the first data value assignment.