To prolong the usable period of a rewritable nonvolatile memory for data backup and reduced a maintenance cost by updating the storage contents of the nonvolatile memory according to the latest data only when the latest data are different from last data.

The latest self-diagnostic data 4a are recorded temporarily in a volatile memory 5. The self-diagnostic data are inputted to a rewrite event occurrence decision means 6 as well. Once the self-diagnostic data 4a are inputted to the decision means 6, the decision means 6 knows that a self-diagnosis is taken at the point of time. These data are data based upon the latest self- diagnosis, so the decision means 6 compares the self-diagnostic data 4a (same as the data temporarily recorded in the volatile memory 5) with the last data stored in the nonvolatile memory 8 and sends a signal 6a to a nonvolatile memory access means 7 only unless they match each other, and the access signal 7 updates the storage contents of the nonvolatile memory 8 with a signal 7a.