I have tried in this paper to clarify Nishi Amane's theory of human nature in the "Hyakuichishinron." This book is said, though uncertain, to have been written during Nishi's stay in Holland (1863-1865), although it was actually published in 1874. Since we find that Ogyu Sorai had much more influence on Nishi in his earlier ages, and since we find Sorai's influence in the "Hyakuichishinron" more than in his other writings after the Meiji Restoration, we may say that the "Hyakuichishinron" is one of his earliest writings which describe his theory of human nature. Thus, by dealing with the "Hyakuichishinron," we clarify his early theory of human nature. And this is the first of the series of studies in Nishi Amane's development of a theory of human nature as a whole.