PURPOSE: To reduce user's work of a file system by automatically copying contents of a file protected from write to a newly acquired data area in accordance with a copy indication flag at the time of write to this file.

CONSTITUTION: A copy indication flag 2 for the file is provided besides a flag 1 which sets write permission or inhibition to the file. At the time of write to the file protected, from write, the copy indication flag 2 is checked; and if copying is permitted, a new file is generated to copy the file. Then, write to the copied file is performed.