PURPOSE: To easily increse the writing frequency of maximum backup data by sorting E2 ROM to plural blocks and writing into another block when the frequency of writing times exceeds the maximum writing frequency.

CONSTITUTION: E2ROM is sorted to the number blocks equal or larger than a value obtained by dividing the maximum backup data writing frequency which is previously guaranteed for a device by the maximum writing frequency which is previously determined for E2ROM. If the maximum backup data writing frequency is four hundred thousand times and the maximum writing frequency is one hundred times, for example, the number of blocks to divide ≥ four. In the case of dividing E2ROM of KB to four blocks, the respective blocks A to D respectively have the capacity of 2KB and a write counters WC counting the number of writing times are assigned to the respective blocks. When it exceeds the maximum writing times number for one block, the writing and after it are moved to another block.