With a computer system having a host computer and first and second storage apparatuses, the second storage apparatus virtualizes first logical units in the first storage apparatus and provides them as second logical units to the host computer, collects configuration information about each first logical unit, and sets each piece of the collected configuration information to each corresponding second logical unit. The host computer adds a path to the second logical units and deletes a path to the first logical units. The second storage apparatus copies data stored in the first logical units to a storage area provided by the second storage device and associates the storage area with the second logical units.