The problem with recent Japanese studies on Waegu is that they do not consider its correlation with the domestic situation in Japan, especially the military realia in Kyushu, which is geographically adjacent to the Korean Peninsula. Japanese academia is looking for the background and substance of the occurrence of Japanese pirates from the domestic situation of Goryeo (Joseon). To overcome this existing perception, this paper paid attention to the term ‘Podo’ mentioned by the Muromachi Bakuhu in 1377, the third year of King Woo of Goryeo. In June 1377, when the Japanese invasion was the most severe in Goryeo, Goryeo dispatched An Gil-sang to Japan asking for an explanation. In response, Muromachi Bakuhu explained that the reality of Japanese pirates invading Goryeo was ‘Podo’, which means a group that fled the arrest of Bakuhu. This has great implications when considering the domestic situation in Japan at that time. This paper examined the mechanism of the occurrence of Japanese pirates in relation to the background of the times of Japan’s civil war, when the term ‘Podo’ appears in official diplomatic documents. As a result, the following commonalities could be found. Whenever the civil war broke out in Kyushu, the groups that lost the civil war fled to Daemado Island to avoid arrest, and then from there they invaded the Korean Peninsula. Although not all Japanese pirates that invaded the Korean Peninsula can be explained in terms of ‘Podo’, many of the Japanese pirates that occurred during the late Goryeo and early Joseon periods included cases of ‘Podo’ triggered by the Kyushu civil war.