PURPOSE: To guarantee data additionally written before a system goes down or an area becomes insufficient at the time of writing a file even when an unexpected state where a processing can not be continued is encountered at the time of additionally writing the data.

CONSTITUTION: At the time of writing new data in an additional form to the file provided with completion information at the completion point of the file, information for which the completion information is added after the new data is written following the completion information provided at the completion point of the file (S1). When the write of the new data and the completion information is ended, the completion information provided at the completion point of the file is changed to continuation information for indicating the continuation of the file (S2). Since the end of the file is surely the completion information when the write is normally performed in a normal state and at least one piece of the completion information remains in the file even at the time of falling into the state incapable of continuation due to the unexpected system down or the shortage of the preparation area of the file, the valid final updating point of the file is guaranteed by the completion information.