The points of "nomos" and "physis" played an important role in the cross-cultural understanding of the ancient Greeks. As to the origin and historical role played by these notions, F.Heinimann's view is still influential. His view is as follows: (1) The origin of these notions stems from ethnological studies of the various cultures of the time. (2) However, the antithetical way of thought of philosophers at that time (e.g. 'name vs reality' or 'seeming vs being') changed the notions of "nomos" and "physis" into sharp antithetical notions. (3) The Sophists used these notions to oppose 'law and convention' (nomos) to 'human nature' (physis) and accepted the lattter and rejected the former. In this paper, I reexamine his view and propose a new interpretation. The outline of my interpretation is as follows: (1) The origin of these notions was not only derived from the ethnological studies, but also from the thoughts of Protagoras, the Sophist. (2) Protagoras presupposed close interrelations between "nomos" and "physis." (3) This presupposition was shared by many Sophists of the day including Antiphon (whom Heinimann regarede the champion of anti-nomos thought). In my view, ancient Greeks had consistently used these twin notions as effective tools for cultural understanding.