A method of migrating data from an old storage subsystem to a new storage subsystem in a data processing system which comprises host computers and storage subsystems. There is provided a route-changing phase before the data is migrated from the old storage subsystem to the new storage subsystem. In the route-changing phase, each host computer can access both the old and new storage subsystems and the new storage subsystem writes data into the old storage subsystem in response to a write request from the host computer and reads data from the old storage subsystem and sends the data to the host computer in response to a read request from the host computer.