A first storage system includes a first storage area for storing data written by a computer. A second storage system includes a second storage area to which the data stored in the first storage area migrates. A third storage system includes a virtual storage area corresponding to the second storage area. An access request to the virtual storage area is converted into an access request to the second storage area to be issued. A management computer transmits, to the third storage system, an instruction to migrate the data of the second storage area to a third storage area of the third storage system based on a determination result between a value of the data migrated to the second storage area and a predetermined threshold value.