This article is to reconsider the causes of `Imohwabeon`, that is to say the accident which the Crown Prince Sado(思悼世子) was killed by his father King. Hitherto most historians has laid emphasis upon the disparity in character between King and Sado and the mental trouble of the Crown Prince. However I examine the political position of Sado in the political situation at the middle of King Yoˇngjo age instead, and thereby make clear the fact that he was eliminated by Noron(老論) for political reasons in the vortex of political strife between Noron and Soron(少論). King Yoˇngjo had made close Noron and Soron promoting TangPyeong policy(蕩平策). Nonetheless Soron was largely eliminated because of Eulhaeoksa(乙亥獄事) in Year Thirty One of Yoˇngjo, and Noron came into power. Under these circumstances, Yoˇngjo was in anguish about how to deal with the Crown Prince who had adhered to Soron. For this reason, King and Sado became estranged each other and Sado was even taken ill. Noron make use of this disparity between them to eliminate him. Noron informed falsely Yoˇngjo that the Crown Prince harbored ill feeling against him, and King Yoˇngjo killed the Crown Prince in person after all. Comprehensively, `Imohwabeon` was a tragedy resulted from the failure of TangPyeong policy. Since this accident, Yoˇngjo had tried to secure the royal power by depending on Chucksin(戚臣) instead of Bungdang(朋黨) political forces. This was the inevitable choice to stand against the subjects power of Noron and the critical modification of TangPyeong policy.