Computer system, management computer, and cluster control method

This computer system includes a plurality of clusters, each comprising at least one virtual machine, and also includes a management computer. The management computer stores cooperation information indicating a cooperative relationship between the plurality of clusters, and a target threshold set for the cluster group. If the value of a parameter calculated for the cluster group does not satisfy the target threshold, the management computer selects a cluster the scale of which is to be changed, from the cluster group on the basis of the cooperation information, then changes the scale of the selected cluster, thus modifying the cluster group, and calculates the value of the parameter for the modified cluster group.