The diplomatic effort of Goryeo dynasty to solve the Japanese pirates problem has been regarded ineffective by the Japanese and Korean scholars. This perception is based on the false recognition about the Na Heung-You’s special mission to Japan in the year of 1375. But the Muromachi Bakuhu of Japan expressed the willingness to cooperate for solving the pirates problem by letter. It reads as follows. First, Japan has been in the state of civil war for 20 years. Second, rebellious forces were the identity of Japanese’s pirates. Third, Japan’s governmental authority has exerted military effort to solve this problem. Fourth, it was said that if Japan’s governmental authority could establish security of the Kyushu region, there would be more pirates invasion. This letter, because of its contents, exposed their own weakness to Goryeo dynasty and it could bring the armed aggression of foreign country. However, from Goryeo’s standpoint, the delivery of this letter meant that it’s not the sign of national invasion but just a pirates’ action of rebellious forces. Goryeo, which was encountering with military threat by the Yuan(元) and Ming(明) Empire from the north at that time, could feel secure by receiving this letter.