A method of controlling a virtual computer system, the method comprising: obtaining, by a management computer, a load value for each of the plurality of groups, and comparing the load value against a preset threshold; identifying, a group whose load value exceeds the preset threshold as a first group; selecting, a second group from the plurality of groups minus the first group; identifying, as a migration target computer, a given physical computer out of physical computers that run virtual computers allocated to the second group; migrating, virtual computers that are provided by the migration target computer to other physical computers within the second group; changing, settings of the network switch in a manner that enables the migration target computer to operate in the first group; adding, the migration target computer to the first group; and controlling, the migration target computer to run virtual computers of the first group.