A storage system 200 has a storage device 240 providing a plurality of logical volumes 250 used as data storage areas of a host 100 and a plurality of MPPKs 210 executing data IO processes for the logical volumes 250 from the host 100 and, when it is determined whether the MPPKs 210 correlated with the logical volumes 250 must be changed based on a storage function such as local copy set for the logical volumes 250, if it is determined that the change is necessary, the usage rates of processors 212 of the MPPKs 210 are acquired and compared with each other and if it is determined that the comparison result satisfies a predetermined condition, the process cost for the logical volumes 250 is calculated for each of the responsible MPPKs 210 to determine the destination MPPK 210 such that the costs of the MPPKs 210 are equalized.