A first computer is provided that executes a plurality of virtual machines (VMs), a storage device, and a second computer is provided that applies patches to OSs (operating systems) operating upon the VMs to the VMs. The storage device holds storage regions (golden images (GIs)) that store data of the OSs operating upon the VMs at certain time instants, a storage region that stores patches applied to the OSs of the VMs after those certain time instants, and snapshots of the GIs. Patches applied to the OSs of the VMs accessed in the snapshots are stored in the storage pool. The second computer selects, as a GI to be a source of acquisition of snapshots, a GI to which are applied patches of a combination that can be created from patches applied to the OS of some VM, and deletes patches that are patches applied to the selected GI, and that moreover, among the patches applied to that VM, are stored in the storage pool.