To previously confirm that a storage means can not be rewritten by displaying a warning on a display means when a rewrite frequency count value approximates the rewrite frequency limit value of the storage means up to a specific number.

It is judged whether 1st information stored in a 1st information storage area 2 matches 2nd information stored in a 2nd information storage area 4. When both the pieces of information match each other, it is decided that the storage contents of a main storage device 1 are not rewritten and the rewriting decision process is finished. When both the pieces of information do not match each other, it is decided that the storage contents of a main storage area 2 are rewritten and the 1st information stored in the 1st information storage area 2 is copied and stored in the 2nd information storage area 4. Then a count part 5 increases its rewrite frequency count value by one. When it is judged that the rewrite frequency count value of the count part 5 approximates the write frequency limit value of the main storage device 1 up to the specific value, a warning is displayed on a display device 7.