1. A computer system comprising: a plurality of virtual machines each having an OS; a virtualization control unit which manages the plurality of the virtual machines; a memory which is utilized by each OS at least in part as a cache region; and a monitor unit which monitors a utilization status of the cache region by each OS; wherein the virtualization control unit manages an allocation region of the memory with respect to each OS, decides the allocation region of the memory which becomes a collection target among the allocation region of the memory already allocated to each OS, based on the usage status of the cache region notified from the monitor unit, and collects the allocation region of the memory which becomes the decided collection target from the OS which is a current allocation destination.