A management computer monitors an allocated capacity of a source virtual volume and an allocated capacity of a virtual pool having a storage area allocated to a destination virtual volume in a storage, judges whether the allocated capacity of the source virtual volume can be covered in the virtual pool or not, and informs the user of a result that data migration fails when the allocated capacity of the source virtual volume cannot be covered. When the allocated capacity of the source virtual volume cannot be covered, the management computer uses another storage area to increase the capacity of the virtual pool. Accordingly, when data migration is performed between the virtual volumes, capacities of physical areas allocated to the source virtual volume and the destination virtual volume can be grasped so that the data migration can be performed accurately between the source virtual volume and the destination virtual volume.