1. An apparatus for controlling live-migrations of a plurality of virtual machines, the apparatus comprising: a memory configured to store resource-usage state information in association with each of the plurality of virtual machines, the resource-usage state information indicating a change in an amount of resources being used for providing a service; and a processor to: acquire, from each of the plurality of virtual machines, the resource-usage state information when a first live migration of a first virtual machine is started, store, in the memory, the acquired resource-usage state information in association with the each of the plurality of virtual machines, calculate a correlation factor indicating a degree of correlation between first resource-usage state information for the first virtual machine and second resource-usage state information for each of one or more target virtual machines other than the first virtual machine, using the acquired resource-usage state information for the each of the plurality of virtual machines, execute a second live-migration on a second virtual machine that is selected from the one or more target virtual machines and has a positive correlation factor with respect to the first virtual machine, while executing the first live migration on the first virtual machine, wherein the positive correlation factor indicates a close similarity between the first resource-usage state information and the second resource-usage state information, and calculate a transfer time needed for migrating each of the plurality of virtual machines, using a memory-rewriting rate indicating a ratio of a memory-writing time to a unit time, the memory-writing time indicating a time used for writing virtual machine data into a memory area for storing the virtual machine data when migrating the each of the plurality of virtual machines.