1. A network virtualization system comprising: a physical node 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, 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 information contained in the virtual node definition with 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.