Data migration can be preformed between source and target storage subsystems without stopping exchanging data between a host computer and each of the storage subsystems.After a next access path, which should be set instead of a specific access path in an available state among access paths, is set between the host computer and a migration target logical unit via a host port and a target port, the status of the next access path is set to an available state and the status of the specific access path is set to an unavailable state. After the next access path is set to an available state, data is migrated from a migration source logical unit of the source storage subsystem to the migration target logical unit of the target storage subsystem.