Transfer of data and replication of data among several storage devices are executed without imposing a load on the computer connected to the storage system. For example, a first host computer monitors usage rates of transfer groups TG11, TG12 and physical path usage rate on a storage device interconnect physical path 50, and when either a usage rate or physical path usage rate goes above a predetermined value, issues to the first storage device 20 an instruction to allocate a reserved volume Ri 1, R12. The first host adaptor 21 receiving the reserved volume Rh, R12 allocation request allocates reserved volume Ru to transfer group TGI, whereby a virtual transfer volume is formed by transfer volume Til and the reserved volume Ru allocated thereto. As a result, capacity of the virtual transfer volume is increased by the equivalent of the reserved volume Ru capacity, so that additional transfer frames may be stored.