This research discuss about Japan reason to joining the Trans Pacific Partenship (TPP). The purpose of this research is to analyze the reason why Prime Minister, Shinzo Abe decided to joining the Trans Pacific Partnership (TPP) and stand even United State plans to exit from Trans Pacific Partnership (TPP). This research use rational actor theory. Based on research that has been hold, the writer found that Prime Minister, Shinzo Abe decided to joining Trans Pacific Partnership (TPP) because two things, first Trans Pacific Partnership (TPP) can increase an existence of Japan’s economy in East Asia and Trans Pacific Partnership (TPP) can also increase Japan’s investment.