It has been impossible in prior art to copy every piece of data that is stored in a virtual volume. A management computer of this invention receives a copy request that designates a real storage area as a copy source, selects a virtual storage area that is associated with plural real storage areas as a copy destination virtual storage area, and judges whether or not a copy source virtual storage area and the copy destination virtual storage area are allocated to real storage areas. Based on a result of the judging, the management computer gives an instruction to copy data that is stored in the copy source real storage area to a copy destination real storage area, and keeps information about a replication relation between the copy source real storage area and the copy destination real storage area on which the data copy has been performed.