A design assistance device includes: a design information storage section configured to store design information containing at least information of slave devices and information of a topology in a network system in accordance with a design created by a user; an actual configuration information generation section configured to generate actual configuration information containing at least the information of the slave devices and the information of the topology in the actual network system; a comparison section configured to compare the design information and the actual configuration information; and an output section configured to generate a comparison screen indicating the respective configurations of the designed network system and the actual network system along with their commonalities and differences and outputting the comparison screen to a display device. Thus, a technology can be provided which assists the user to easily compare the designed network configuration and the actual machine's network configuration.