A processor of a NAS performs a calling process of a local file system based on an operation request of a file from an application, the local file system processes the operation request of the file, an IO Hook program performs an update process of state management information of the file based on input information with respect to the local file system related to the operation request or an operation content, and a file virtualization program performs a management process of the file between the NAS and a CAS based on the state management information.