A design assistance system for assisting design of a network system in which at least one slave device joins to a communication bus drawing a line, tree, or star topology from a master device as a starting point, the network system being formed by interconnecting ports of each of the master and slave devices, the design assistance system comprising: a display configured to display a design assisting GUI; a control device configured to control the GUI which is displayed on the display; and an input device configured to permit a user to input a command configured to operate the GUI, wherein the control device comprises: a storage section configured to store slave information data in which unique information of the slave device is described; an acquisition section configured to acquire information of a connection interface of a port of the slave device from the slave information data; an a generation section configured to generate 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 generate a design assisting GUI having a topology display screen configured to display the topology of the network system under design which is built by connecting the device components to each other with the line component. a display configured to display a design assisting GUI; a control device configured to control the GUI which is displayed on the display; and an input device configured to permit a user to input a command configured to operate the GUI, wherein the control device comprises: a storage section configured to store slave information data in which unique information of the slave device is described; an acquisition section configured to acquire information of a connection interface of a port of the slave device from the slave information data; an a generation section configured to generate 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 generate a design assisting GUI having a topology display screen configured to display the topology of the network system under design which is built by connecting the device components to each other with the line component. a storage section configured to store slave information data in which unique information of the slave device is described; an acquisition section configured to acquire information of a connection interface of a port of the slave device from the slave information data; an a generation section configured to generate 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 generate a design assisting GUI having a topology display screen configured to display the topology of the network system under design which is built by connecting the device components to each other with the line component.