To provide a network design system in which each designer does not need to set a traffic parameter and a calculation amount (time) is reduced.

The network design system designs a network interconnecting nodes based on given node arrangement information and inter-node traffic information. In this case, based on the node arrangement information and inter- node traffic information received from an entry section 300, the system has a design section 400 that decides a link configuration of the network interconnecting nodes and a link speed to give the decided configuration and speed to a calculation section 500 and the calculation section 500 that calculates the network cost and the quality calculation based on the link configuration, the link speed and the entered inter-node traffic information.