Tokyo , officially Tokyo Metropolis , is one of the 47 prefectures of Japan and has been the capital since 1869. The Greater Tokyo Area is the most populous metropolitan area in the world. It is the seat of the Emperor of Japan, the Japanese government and the National Diet. Tokyo is in the Kantō region on the southeastern side of the main island Honshu and includes the Izu Islands and Ogasawara Islands. Tokyo was formerly named Edo when Shōgun Tokugawa Ieyasu made the city as his headquarters in 1603. It became the capital after Emperor Meiji moved his seat to the city from Kyoto in 1868; at that time Edo was renamed Tokyo. Tokyo Metropolis was formed in 1943 from the merger of the former Tokyo Prefecture and the city of Tokyo .