Each time the file access request is received, it is discriminated whether there is an ACL which corresponds with the file system of the type following the switching in the file designated by the file access request. In cases where the judgment result is that there is no such ACL, an ACL which corresponds with the file system of the type following the switching is generated on the basis of an ACL which corresponds with the file system of the type prior to the switching which is associated with the designated file, and the ACL thus generated is used to control access in accordance with the received file access request.