PURPOSE: To check the contents of a backup memory automatically, inform an operator of abnormality occurrence, and reduce the load on a user by inspecting the contents of the backup memory at prescribed timing and displaying the inspection result on a display means.

CONSTITUTION: The backup memory A is checked by a storage inspecting means B at the prescribed timing, and the result is displayed on the display means C. The display means may display all stored erroneous data for a check or use an alarm tone of a buzzer, etc., or light emitting means.