A computer system connected to an external computer issuing a data input/output request and a storage device, includes a logical storage area of a first layer of the storage device and a logical storage area of a second layer of the external computer. The logical storage area of the first layer includes a common area and an individual area. A storage area of the common area is associated with one or more of the second layer's logical storage areas. A storage area of the individual area is associated with one storage area of the second layer's logical storage area. The computer system performs duplication determination on a plurality of data sets and associates a plurality of data sets determined as being duplicated with the common area to associate the storage area of the common area with a plurality of storage areas of the logical storage area of the second layer.