Used is a nonvolatile memory such as a multivalued NAND flash memory having memory cells for holding the data of a plurality of pages. When the data is to be written in the nonvolatile memory (110), aphysical unit is constituted at the unit of a plurality of paired pages. When all the physical units cannot be written, the data is copied from the old physical block holding the already written effective data, and is written in a new physical block till the end of the physical unit. The data can be written, when next written, from the first of a new physical unit, so that the error can be prevented.