Both in Nihon-shokki and Shotoku-taishi-denryaku is found a description of several attempts to invade Shiragi, a country in the Korean Peninsula, which Japan made in the age of Prince Shotoku. According to the two ancient writings, Japan was at first successful in invasion but eventually failed because the general died of illness in Tsukushi, Kyushu before sending more forces. Later in the medieval legend of the prince, however, the story was almost surprisingly changed. There Prince Kime, Prince Shotoku's younger brother, appears as a hero who played a vital role in triumphantly defeating Shiragi. Moreover, the legend is extremely nationalistic as it justifies Japan in its invasion on the ground that it was for the defense of the country which was then under the threat of Shiragi. The aim of this essay is thus to examine how the Shintoistic nationalism of the Middle Ages is reflected and represented in the Prince Shotoku legend.