A physical server of a virtual machine (VM) priority control system transmits priority group setting information in which the amounts of resource usage of VMs and priority groups to which the VMs belong are stored to a controller. The controller determines a priority group to which each VM is desired to belong on the basis of an acquired amount of resource usage of the VM, generates dummy VMs corresponding to vacancies belonging to each group, and determines whether performance of each VM can be guaranteed. Further, the controller determines a new current priority group of the VM by exchanging the current priority group of the VM with the current priority group of a pair whose priory groups are paired with those of the VM among pairs of the current priority group and the desired priority group of each of the VMs and the dummy VMs.