PURPOSE: To make it possible to detect a power failure, etc., securely by exercising a supervision over a flag area by using the specific area of a bubble memory unit as the flag area and by writing fixed information as memory contents are modified.

CONSTITUTION: Once correction information is inputted from console panel 5, CPU4 executes editing processing process in memory 3. In this case, bubble memory 1 is provided with flag area 1b. To modify the memory contents of memory 1, CPU4 writes modification start information in flag area 1b before the start of contents modification and then modification end information after the end of the modification. Then, CPU4 exercises a supervision over the contents of a flag to detect an electric power failuer, etc., occurring during the modification of memory contents. As a result, malfunction can be prevented.