The primary purpose of this research is to analyze the formation of international custom of medieval East Asia under the system of the Great Ming Code. It focuses on Korea’s policies regarding Japanese crimes in the fifteenth century. This study particularly investigates how the Great Ming Code affected the East Asian system of order. We find that Confucianism, which was the basis for the Great Ming Code, had a great influence on the formation of customs in East Asia in such areas as the establishment of patriarchal authority, filial piety, and the five punishments system. This study also investigates how etiquette, which served as a foundation for diplomatic regulations, affected Korea-Japan relations during the fifteenth century. It also analyzes Joseon (Korea)’s control policies against illegal acts committed by the Japanese, who tried to enter Joseon for economic gain, from the perspective of the Great Ming Code.