When an operation request of a file is made, a first storage system performs an operation of the file stored in the first storage system on the basis of the operation request, describes, in an operation log, a counter value of a management information file, associated with the file on which the operation is to be performed, together with operation content with respect to the file, replies to the operation request, and updates the counter value of the management information file when the file stored in the first storage system is transferred to a second storage system.