PURPOSE: To improve file input/output processing efficiency by preventing file areas from being distributed by securing an extended area on a disk device equipped with the largest empty area when extending the files at plural disk devices.

CONSTITUTION: Concerning the file extending method at the time of file input/ output processing, a disk device use condition managing means 22 judges whether it is possible to secure the file extending area inside the disk device where the file exists or not and when it is impossible, a file securing area judging means 25 decides the other disk device equipped with the largest empty area. Further, it is judged whether the copy area of the existent file can be also secured or not and when it is possible, the existent file copy area is secured. An extended file updating means 23 secures and updates the extended file area on this disk device equipped with the largest empty area and after an existent file area copying means 24 copies the file from the existent file area to the existent file copy area, the existent file area is opened.