Occupancy amount of physical resource of a virtual server(VS) is calculated based on maximum physical resource amount indicating performance of a physical server(PS), the occupied virtual resource coefficient indicating relation of physical resource amount used by the VS to the physical resource amount allocated to the VS and the allocated physical resource coefficient indicating relation of the allocated physical resource to the maximum physical resource amount of the PS, and change value of the occupied physical resource amount from a predetermined occupied physical resource amount is calculated based on the calculated occupancy amount and the predetermined occupied physical resource amount. The migration time required of the VS is calculated based on the calculated change value, variation ratio indicating degree of influence exerted by change of the occupied virtual resource coefficient of the VS on the required migration time and reference execution time set based on the predetermined occupied physical resource amount.