The data migration method of this invention is for a computer system. The data migration method is characterized in that the storage system measures, for each logical volume, a used I/O amount and a storage capacity of the logical volume, the management computer manages the logical volumes in groups by storing the association relation between the logical volumes and the groups, the management computer obtains, from the storage system, the used I/O amount of the logical volume and storage capacity, which are measured by the storage system, the management computer identifies a logical volume to which data is to be moved based on the ratio of the obtained I/O amount of the logical volume to the obtained storage capacity of the logical volume, and the management computer instructs the storage system to move data from the data's original logical volume to the logical volume designated as the migration destination.