American and Japan launched the military intervention in Siberia of the Soviet Union during the latter half of the First World War. Much of their interrelations in the operation is yet to be studied. Thus, this paper is divided into five parts: the first section explains why I write the paper; the second section illustrates Japan's consideration of the intervention in the light of the whole international context; the third section treats the fluctuation of American policy towards the Japanese intervention; the fourth section analyzes American and Japanese involvement in the Chinese Eastern Railway; the last section is the concluding part.