1. A storage system comprising: a plurality of controllers connected to each other via a bus, wherein each of the controllers includes a plurality of cores in charge of processing of a plurality of logical units, and each core sets a relevant logical unit as an access object for a host computer and controls sending/receiving of data to/from one or more disks in association with read/write from/to the relevant logical unit (LU), wherein each of the controllers is configured to: (a) extract, for each of the logical units, patterns of a combination of a core as an LU ownership transfer source having an ownership for the logical unit and a core, other than the LU ownership transfer source, as an LU ownership transfer destination candidate to which the LU ownership for the logical unit is transferred, based on LU ownership management information defining correspondence relationships between the logical units and the cores having LU ownerships for the logical units; (b) measure, for each of the logical units, an usage of the cores and other resources constituting the respective controller in the data processing for the respective logical unit; predict, for each of the logical units, based on the measurement results, a change in the usage of the LU ownership transfer destination candidate core and the resources after the LU ownership transfer to the LU ownership transfer destination candidate core; (c) predict an overhead corresponding to a change in the usage of the resources that occurs in the LU ownership transfer from the LU ownership transfer source core to the LU ownership transfer destination candidate core; and (d) select a combination pattern that matches setting information including one or more policies a user sets for load balance, from among the respective patterns extracted, based on the respective prediction results of the prediction in (b) and (c), transfer the LU ownership to the LU ownership transfer destination candidate core belonging to the selected combination pattern, and register the selected combination pattern of cores in the LU ownership management information, and the controllers execute the LU ownership transfer between cores existing in different controllers as well as between cores existing in the same controller and the LU ownership transfer between the controllers is included in the LU ownership transfer for which the prediction of a change in the usage of the resources is performed.