To response to the end of Cold War and global anti-terrorist war, U.S. implements global military realignment; therefore, U.S. realigns its forces in Japan under the U.S.-Japan alliance framework. This paper starts with the review of roles and functions of the alliance in the past. Later, it analyzes Japan's future security strategies in the post-Koizumi era through three system layers: security environment of East Asia, Japan's domestic politics, and Japanese leaders' intentions. Finally, it explores Japan's security roles and U.S.-Japan alliance's functions and its changes in the future.