A storage device is provided with a file I/O interface control device and a plurality of disk pools. The file I/O interface control device sets one of a plurality of storage hierarchies defining storage classes, respectively, for each of LUs within the disk pools, thereby forming a file system in each of the LUs. The file I/O interface control device migrates at least one of the files from one of the LUs to another one of the LUs of an optimal storage class, based on static properties and dynamic properties of each file.