1. A computer-readable, non-transitory medium storing a virtual machine management program causing a computer, which is able to communicate with two or more servers each on which a virtual machine operates, to execute a process, the process comprising: collecting information, from which a physical resource capacity currently used by each virtual machine running on the each server is able to be specified; calculating a reserve capacity for each virtual machine based on the physical resource capacity currently used by each virtual machine which is specified from the collected information, the reserve capacity being a physical resource capacity that is expected to be further required for each virtual machine to operate; selecting at least one of the virtual machines operating on each of the two or more servers as a target virtual machine to be migrated, and selecting a destination server to which the target virtual machine to be migrated is migrated among the two or more servers so that a difference between the reserve capacity for a virtual machine running on the destination server and the reserve capacity for the target virtual machine to be migrated requires is less than a difference between a reserve capacity for a virtual machine running on another server and the reserve capacity for the target virtual machine to be migrated; and migrating the target virtual machine to be migrated to the destination server.