A computer system having a first storage system which includes a first logical volume and a second logical volume, wherein the second logical volume stores a first differential data item representing a differential between data that was stored in the first logical volume at a first time instant and data that was stored in the first logical volume at a second time instant being later than the first time instant, and a second storage system which includes a third logical volume. The first storage system transmits the first differential data item to the second storage system, the second storage system stores the first differential data item into the third logical volume, the first storage system deletes the first differential data item from the second logical volume, and the computer system retains management information set for associating the first logical volume with the third logical volume storing therein the first differential data item.