Abstract: New data is appended to a file having a first end-of-file information at the end thereof, and even in the case where a hazardous state such as a system failure or shortage of storage area unexpectedly occurs during data write operation, the data in the file which existed before the occurrence of the hazard can be read. First, new data having a second end-of-file information at the end thereof is written subsequently to the first end-of-file information of the file, and when the writing of the new data with the second end-of-file information is completed, the first end-of-file information of the file is altered to continuation information indicating continuity of the file. Accordingly, even if the write operation unexpectedly fails due to system failure or shortage of storage area, at least one end-of-file mark exists in the file, whereby the effective last update point can be specified by the second end-of-file information.