1. A storage system comprising; a plurality of storage devices, wherein a plurality of logical units (LU) is configured in physical storage areas of the storage devices; and a plurality of controllers connected to each other via a bus, in which each of the controllers includes a plurality of processing cores in charge of input/output processing of input/output to/from the plurality of logical units (LU); wherein each processing core is configured to be in charge of input/output processing of input/output to/from a respective logical unit, to set the respective logical unit as an access object for a host computer and to control sending/receiving of data to/from one or more storage devices in association with read/write from/to the respective logical unit; and wherein each of the controllers is configured to: (a) extract, for each of the logical units, a plurality of possible LU ownership change patterns based on LU ownership management information, which defines correspondence relationships between the logical units and the processing cores having LU ownerships for the logical units, respectively, in that the correspondence relationships indicate which processing core is currently in charge of processing with regard to each logical unit, wherein each possible LU ownership change pattern indicates a combination of the processing core, which is currently in charge of the respective logical unit, as an LU ownership transfer source and another processing core as an LU ownership transfer destination candidate to which an LU ownership for the respective logical unit can be transferred by means of an LU ownership transfer; (b) measure, for each of the logical units, the usage of resources in the data processing for the logical unit, wherein the resources include the plurality of processing cores and other resources constituting the controllers; (c) predict, for each of the logical units and each possible LU ownership change pattern for the respective logical unit, a change in the usage of the resources, including the respective LU ownership transfer destination candidate core of the respective possible LU ownership change pattern, after LU ownership transfer from the respective LU ownership transfer source core to the respective LU ownership transfer destination candidate core based on the measurement results in (b); (d) predict, for each of the logical units and each possible LU ownership change pattern for the respective logical unit, a respective overhead corresponding to a load to be generated when executing transfer processing in the LU ownership transfer from the respective LU ownership transfer source core to the respective LU ownership transfer destination candidate core; and (e) select one of the plurality of extracted LU ownership change patterns which matches setting information including one or more policies a user sets for load balance based on the respective prediction results of the predictions in (c) and (d); and (f) execute the LU ownership transfer from the respective LU ownership transfer source core to the respective LU ownership transfer destination candidate core belonging to the selected LU ownership change pattern; and (g) register the selected LU ownership change pattern in the LU ownership management information; wherein the controllers are configured to execute the LU ownership transfer in (f) between cores existing in different controllers are well as between cores existing in the same controller; and wherein the LU ownership change pattern corresponding to the LU ownership transfer between the controllers is included in the possible LU ownership change patterns for which the prediction of change in the usage of the resources is performed.