Memory corruption can be suppressed. When data stored in a random access area are read, the read data (physical block) are retrieved by a logic block number and newest data are read by referring to an incremental counter of data having that logic block number. When data are stored in the random access area, the incremental counter and the logic block number of data already stored in the random access area are referred and a physical block set to be unnecessary is set to a writer buffer, and then the data are written to this write buffer.