Difference data reception unit receives difference data for all segments and stores it in nonvolatile memory. Restoration processing unit stores restoration process segment number indicative current process segment in the nonvolatile memory, and then restores segment data from difference data and stores it in nonvolatile memory. Overwrite processing unit stores overwrite processing segment number indicative of immediately preceding process segment nonvolatile memory, and then reads restored data which is restored on the immediately preceding segment from nonvolatile memory and overwrites it onto the data to be written of the volatile memory. If the power supply is interrupted during restoration process of segment data, a resuming processing unit resumes restoration process from the head of the segment of the restoration process segment number and if the power supply is interrupted during the overwriting process of the segment data, resumes the overwriting process from the head of the overwrite processing segment number.