PURPOSE: To surely perform the backup of data even when a data transfer is interrupted.

CONSTITUTION: When a preservation key is operated, the abnormalities in a main memory and a sub-memory are checked (S2, S4). If the memories are normal, the preserving processing of data is executed (S6). Then, whether the preserving processing is interrupted or not is discriminated (S7), '1' is set to a flag Po when the processing is interrupted (S8) and the message to the effect that data can not be preserved is displayed (S5). Similarly, when the calling of data from the sub-memory is performed by operating a calling key, whether a calling processing is interrupted or not is discriminated (S15). When the processing is interrupted, '1' is set to a flag P1 (S16). At time of turning ON a power source, whether the flag P0=1 is right or not and whether the flag P1=1 is right or not are discriminated (S21, S23). If the flag is '1', the preservation processing or the calling processing is executed (S22, S24).