A management server and a data migration method enabling a storage apparatus to be replaced while retaining data consistency and without halting access by a host apparatus are proposed.In a computer system which comprises a first primary storage apparatus and a first secondary storage apparatus and a second primary storage apparatus and a second secondary storage apparatus, a first virtual volume of the second primary storage apparatus is externally connected to a first primary volume of the first primary storage apparatus, a total cache-through mode is configured as a cache mode in a case where a read command is supplied by the first host apparatus, unique information for the first primary volume is configured for the first virtual volume, a path to the first primary volume is switched from the first host apparatus to a path via the first virtual volume, and a second primary volume in the second primary storage apparatus is configured to form a copy pair with a second secondary volume in the second secondary storage apparatus.