It is desirable that data stored in an old storage device is migrated to a new storage device without any interruption. According to a computer system of this invention, in a first data storage device, a second data storage area of a second data storage device is recognized as a virtualized data storage area of the first data storage device, a data copy relation between the first data storage area and the virtualized data storage area is established, the second data storage area is recognized from the computer based on the data copy relation, data is copied from the first data storage area to the virtualized data storage area based on the data copy relation, and after completion of a data copy operation, a connecting path is switched from the first data storage area to the second data storage area based on the data copy relation.