Analizing the educational idea of Horace Mann (1796-1859) from two viewpoints, that is, educational interest and political one, I elucidate a problem with which everybody may confront in the process of thinking about education in a society and a nation. As an educational interest, I will present Mann's interest in child's development and growth according to his idea of educatee, theory of teaching and educational career. Also, as a political interest I would like to manifest his interest in the relationship between education and a nation or a society. It would be discussed through the both actual side, that is, market value of school, formation of American nationality and recommendation of human physiology which reflect his period and his society, and ideal side emphasized by him. His educational interest specially pays attention to the process of child's development and growth, and his political one does to its result. Therefore, it might be said that these two kinds of interest are paradoxical. Through the idea of the improvability of the race which are supported by science of phrenology and Unitarianism, Mann exactly tried to solve the problem of the conflict of these two paradoxical interests. However, was this problem truly solved ? It must be investigated again depending on the idea of "goodness".