The first aim of this paper is to clarify that Mencius who considers human nature to be good, and thus is against any wars. Secondly, this paper discusses the anti-war theory of Toegye who is considered to follow Mencius' way correctly. Thirdly, this paper is to show that King Hyo-Jong who planed to send the troops to conquer the north has to be considered as just war theorist and thus against Mencius. Mencius stated that there is no just war in the bible Spring-Autumn. And he even went further that every war is unjust. His anti-war theory is based on his respecting life. Toegye claimed that Choseon dynasty should enter into friendly relations with Japanese of Tsushima island to protect thte lives of Choseon citizen. On the other hand, King Hyo-Jong planed to send the troops to conquer the north to revenge for shame he earned during being taken hostage in Chung dynasty for 8 years, although the life of Choseon citizen was very harsh and almost unbearable at that time. Mencius said earlier that citizen is the most precious, country the next, king the least. King Hyo-Jong is the monarch of country of Confucianism, but he is certainly against the teaching of Mencius, because he respects his retaliatory spirit more than life of Choseon citizen. We can conclude that anti-war theory of Toegye coincides with that of Mencius and thus true way of Confucianism.