Provided is a design assistance system that enables even a user having no expert knowledge to easily design a network system taking into consideration the types of port connection interfaces. The design assistance system has a storage section for storing slave information data in which unique information of the slave device is described; an acquisition section for acquiring information of a connection interface of a port of the slave device from the slave information data; and a generation section for generating a device component serving as a GUI component representing the slave device based on the slave information data, generates a line component serving as a GUI component representing connection of the ports of the slave devices in different aspects in accordance with different types of the connection interface, and generates a design assisting GUI having a topology display screen configured to display the topology of the network system.