PURPOSE: To improve the reliability of the titled system, by automatically restoring the system by means of one backup memory belonging to a system which is not diagnosed out of doubled backup memories, even when the automatic restoring process of the system is started under a condition where the content of the backup memory being diagnosed is rewritten.

CONSTITUTION: A program for automatically restoring a system 7 and service program 8 are stored in a backup memory 3. When a diagnosis related to the backup memory 3 is performed, the writing position in the backup memory 3 is limited in the storing address of the automatic system restoring program 7. When this arrangement is made, the automatic system restoring program 7 stored in the backup memory 3 being diagnosed does not make normal operations and an automatic system restoring function by the backup memory 3 being diagnosed is stopped. Therefore, the automatic system restoring process is not started under a condition where the content of the backup memory 3 being diagnosed is rewritten and, as a result, the reliability of the system is improved.