The virtual computer system comprises a network device which is capable of being coupled via a network to an external network apparatus and is able to operate multiple virtual computers. The network device comprises multiple communication ports which can be provided to a virtual computer with respect to a single physical communication port. When a certain the above-mentioned virtual computer is booted up, the virtual computer system acquires a MAC address corresponding to the above-mentioned certain virtual computer from multiple MAC addresses, which correspond to each of multiple the above-mentioned virtual computers and are allocated to the virtual communication port provided in the virtual computer, and configures this MAC address as the MAC address of the virtual communication port allocated to the above-mentioned certain virtual computer.