1. A logical volume rearranging method used in a storage network environment including at least one storage subsystem and at least one server, said at least one storage subsystem including means for moving a logical volume in said at least one storage subsystem to a parity group in said at least one storage subsystem or to a parity group in a different storage subsystem, said method comprising the steps of: acquiring server/storage mapping information of said at least one storage subsystem from each said at least one server and acquiring performance information of said logical volume and said parity group from each said at least one storage subsystem; acquiring a required I/O performance and a rearranging rule for said logical volume; determining a rearranging destination parity group of said logical volume designated by a user, based on said server/storage mapping information, said performance information, said required I/O performance, and said rearranging rule, wherein the rearranging destination parity group is determined so as to optimally reduce the required I/O performance of a predetermined high priority application; and balancing I/O load between parity groups in said storage network environment by moving said logical volume to said rearranging destination parity group based on said server/storage mapping information, said performance information, said required I/O performance, and said rearranging rule.