A method of balancing a load in a computer system having at least one storage system, and a management computer, each of the storage systems having physical disks and a disk controller, the load balancing method including the steps of: setting at least one of the physical disks as a parity group; providing a storage area of the set parity group as at least one logical volumes to the host computer; calculating a logical volume migration time when a utilization ratio of the parity group becomes equal to or larger than a threshold; and choosing, as a data migration source volume, one of the logical volumes included in the parity group that has the utilization ratio equal to or larger than the threshold, by referring to the calculated logical volume migration time, the data migration source volume being the logical volume from which data migrates.