1. A method for executing migration in an information processing system including: at least one server system that accesses a storage apparatus in response to a request transmitted from a client device; and a plurality of virtual servers which are implemented in the server system, the method allowing execution of migration between the plurality of virtual servers, the method allowing the server system to execute the steps of: storing an amount of resources of the server system that are usable for executing migration, storing a plurality of policies each defining how to execute a corresponding one of a plurality of migrations, and including information for specifying a resource use amount of the server system to be used to execute the migration, for each of the policies, obtaining the resource use amount needed to execute the migration according to the policy, allocating an amount of usable resources to each of the policies in accordance with the resource use amount obtained for the policy, in the migration according to each of the policies, allocating a certain amount of resources usable for executing the migration, to the virtual server which serves as either of a migration destination and a migration source, the certain amount of resources being determined within the amount of resources allocated to the policy, and executing the migration according to each of the policies in accordance with the resource use amount allocated to the policy.