A wide area distributed storage system checks for erasure of WORM (Write-Once Read-Many) files. A first management server acquires a storage location of a second file on a second disk apparatus which corresponds to a first file stored on a first disk apparatus, of which a retention period has expired, and queries a second management server about a retention period of the second file whose storage location has been acquired. The queried second management server refers to a file table and indicates existence or non-existence of the second file on the second disk apparatus and whether the retention period of the second file has expired or not. The system avoids unintended deletion of files by querying a plurality of file management servers about whether a file with an expired retention period should be set as a deletable object.