Abstract: When a rewriting instruction, which designates a specified rewriting bank is received, a rewrite processing unit copies data of the rewriting bank into a backup bank and, thereafter, writes new data prepared in a work memory such as an RAM or the like and clears the backup bank after completion of the writing operation. In this instance, flag information indicating whether data is valid or invalid is set into a plurality of banks of a flash memory. A restarting process after the writing process was interrupted is recovered by checking the flag information in all of the banks before the start of the rewriting operation and by executing the rewriting process from the beginning or in the middle of the process.