PURPOSE: To attain long time backup by storing the sum of a fixed pattern and necessary data in an idle address of a memory or an area of a working memory when voltage supply from a main power supply is interrupted.

CONSTITUTION: When the main power supply is turned off during an operation, voltage is supplied to a microcomputer 1 by the discharge of a backup capacitor 4. At that time, the microcomputer 1 stores the sum of the fixed pattern having previously fixed random number and the contents of data requiring backup in the idle address of the memory 17 and the memory address in the working area which is not used in the time other than real operation. When voltage is supplied from the main power supply 13 again, a reset signal is impressed from a node between a capacitor 20 and a resistance 21 to the microcomputer 1. If the backup is executed in the operation voltage range of the memory 17, the contents are held as they are even if the backup is reset on the way and then the operation starts again, so that the contents are guaranteed.