1. A network virtualization system comprising: a physical node having a memory constituting a network; and a domain management apparatus that instructs the physical node about a configuration of a virtual node and a virtual link of a virtual network in order to allocate a resource in the physical node, wherein when the physical node assigns a virtual machine to the virtual node based on a definition of the virtual node contained in an instruction from the domain management apparatus, the physical node writes a configuration associating a check result indicating that the resource can be allocated, information contained in the virtual node definition, and information about the virtual machine into a configuration file of an operating system to be started up on the virtual machine before start-up of the virtual machine and, after that, starts up the virtual machine, wherein the physical node: associates a virtual network interface card (NIC) of the virtual machine with each of a virtual interface included in the virtual node definition one by one as information included in the virtual node definition; assigns a value to a media access control (MAC) address of the virtual network interface card (NIC) as information about the virtual machine, the value being unique at least within the virtual machine; and writes a rule associating the virtual interface included in the virtual node definition with a media access control (MAC) address assigned to the virtual network interface card (NIC) of the virtual machine into a configuration file of an interface mapping unit of an operating system to be started up on the virtual machine in a virtual disk of the virtual machine before start-up of the virtual machine and, after that, starts up the virtual machine.