A method for moving a virtual machine is provided which considers an input/output between array groups and virtual machines after being moved. The computer system includes a physical server executing the virtual machines, a storage device having array groups, and a management server that manages a link between the virtual machines and the array groups. The computer system is characterized in that an estimated I/O volume of the virtual machine after being moved is calculated and that, if the estimated I/O volume exceeds the maximum I/O volume of the array group connected to the moved virtual machine, a search is made to find an array group whose maximum I/O volume is smaller than the estimated I/O volume.