1. A non-transitory computer-readable medium storing a management program when executed by a computer performs: collecting amount information of a plurality of servers, the amount information being a physical resource amount currently used by virtual machines running on the servers; calculating, for a target server, of the servers, a reserve amount of a physical resource required by a virtual machine running on the target server by obtaining the physical resource amount currently used by the virtual machine from the collected amount information and subtracting the obtained physical resource amount currently used by the virtual machine from a physical resource amount that is expected to be required for the virtual machine to run; calculating, for the servers, an average of reserve amounts of a physical resource required by virtual machines running on a server of the servers; and selecting a target virtual machine running on the target server, and selecting a destination server, of the servers, different from the target server, the target virtual machine of the target server to be migrated to the destination server, wherein the destination server is selected based on a difference between the reserve amount of the physical resource required by the target virtual machine and the average of the reserve amounts of the physical resource required by the virtual machines running on the destination server, the difference being less than the difference between the reserve amount of the physical resource required by the target virtual machine and the average of the reserve amounts of the physical resource required by the virtual machines running on another server different from both the target server and the destination server.