I have tried in this paper to clarify Nishi Amane's attempt at unified science in the early Meiji Era. The article consists of four parts : 1) Nishi's cultural background for unified science ; 2) Nishi's attempt at unified science in the "Hyakuichishinron ;" 3) Nishi's attempt at unified science in the "Hyakugakurenkan;" 4) Nishi's attempt at unified science in the "Seiseihatsuun." In the first part, we deal with Nishi's cultural background before he was confronted with European philosophy. We find that Nishi's studies of Chu Hsi's and Ogyu Sorai's philosophy, together with his medical training, provided the appropriate foundations for his unified science. In the second part, we concern Nishi's attempt at unified science in the "Hyakuichishinron (1866-67)" and find confucianistic influences in his attempt, together with Auguste Comte's and J. SMill's. In the third part, we discuss his attempt at unified science in the "Hyakugakurenkan (1870)" and deal with his principles on which he tried to build up his system of unified science. In the fourth part, we examine his attempt to bridge over the gap between physiology and psychology in the "Seiseihatsuun (1871-73)." This gap was considered to be an insurmountable one between mental and physical sciences in those days. We discuss Nishi's attempt at construction of a bridge over this gap and deal with his frustration in this attempt.