Time required for a file server to determine files to be migrated from a first storage region to a second storage region is reduced. A file service system coupled to the storage system for providing a first storage region and a second storage region stores identifiers of files that have been accessed by a client, in accessed file identifier information and determines the files to be migrated from the first storage region to the second storage region based on metadata of the files stored in the first storage region and the accessed file identifier information.