The present invention allows load balancing between processor units without impacting the I/O performance of the storage system. An LDEV owner right is changed on the basis of static information that does not dynamically change in accordance with the number of I/O commands relating to a LDEV. This information is a load index determined for each LDEV. Any of a plurality of processor units selects a processor unit that is to be an assignment destination of the owner right of a target LDEV, based on the load index that has been assigned to each processor unit and the load index of the target LDEV, and assigns this owner right to the selected processor unit. The load index assigned to the processor unit is a value based on the load index of one or more LDEV respectively corresponding to one or more owner rights assigned to this processor unit.