1. A storage system comprising: a plurality of processor units, each of which includes a processor that processes an I/O (Input/Output) command from an external device; and a plurality of storage devices providing one or more RAID groups (VDEVs), each of which including a plurality of logical volumes (LDEVs), the storage devices storing a load index determined for each of the LDEVs, wherein each of the processor units has at least one owner right for one of the LDEVs, wherein the load index of each of the LDEVs is a static value that does not dynamically change in accordance with the number of I/O commands to the LDEV, wherein when adding a new LDEV, any of the plurality of processor units, based on the following (X1) and (X2): (X1) the load index assigned to each of the processor units; and (X2) the load index of each of the LDEVs, selects, from among the processor units, a processor unit which has the lowest load index (X1) as an assignment-destination of an owner right for the new LDEV, and assigns this owner right to the selected processor unit, wherein the (X1) load index of the processor unit is sum of the load indexes of one or more LDEVs (X2) corresponding to one or more owner rights that have been assigned to the processor unit, and wherein, the (X2) load index of each of the LDEVs is determined by the following formula: 
 ( X 2)=(an average time per an I/O process)×(drive performance)×(the number of storage drives serving as the basis for one of the VDEVs that constitutes the basis of the LDEV)/(the average number of drive accesses per I/O process)/(the number of one or more LDEVs included in the VDEV that constitutes the basis of the LDEV).