1. An virtual machine monitor for monitoring virtual machines running on a computer system which runs a plurality of virtual machines by managing relation between a physical memory and a plurality of virtual memories, said virtual machine monitor comprising: a mapping data generator for determining a virtual page number capable of identifying a virtual page of a virtual memory where an executable code in an object file of a guest OS executed on a virtual machine is mapped; a virtual page manager for managing a virtual page record containing the virtual page number, a first object ID capable of identifying a part of the object file corresponding to the virtual page number, and a detection flag indicative of existence of another virtual page record containing a second object ID identical with the first object ID; a machine page manager for managing a machine page record containing the first object ID, and a machine page number capable of identifying a machine page of the physical memory where a virtual page identified with the virtual page number is mapped; a sharable page detector for determining, upon accessing an access page of the virtual memory, whether a detection flag of the virtual page record containing a virtual page number of the access page indicates existence of another virtual page record containing the second object ID identical with the first object ID, and determining, when the detection flag indicates existence of another virtual page record containing the second object ID identical with the first object ID, whether a machine page number of the machine page record containing the second object ID has been set; and a page mapper for mapping a set machine page number to the virtual page number of the access page upon determining that the machine page number has been set.