Japan is a constitutional monarchy where the power of the Emperor ( literally "heavenly sovereign") is very limited. As a ceremonial figurehead, he is defined by the constitution as "the symbol of the state and of the unity of the people". Power is held chiefly by the Prime Minister of Japan and other elected members of the Diet, while sovereignty is vested in the Japanese people. The emperor effectively acts as the head of state on diplomatic occasions. Akihito is the current Emperor of Japan.

Japan's legislative organ is the National Diet, a bicameral parliament. The Diet consists of a House of Representatives, containing 480 seats, elected by popular vote every 4 years or when dissolved, and a House of Councillors of 242 seats, whose popularly-elected members serve six-year terms. There is universal suffrage for adults over 20 years of age, with a secret ballot for all elective offices. The liberal conservative Liberal Democratic Party (LDP) has been in power since 1955, except for a short-lived coalition government formed from opposition parties in 1993. The largest opposition party is the social liberal Democratic Party of Japan.

The Prime Minister of Japan is the head of government. The literal translation of his Japanese title is "Prime Minister of the Cabinet". The position is appointed by the Emperor of Japan after being designated by the Diet from among its members, and must enjoy the confidence of the House of Representatives to remain in office. The Prime Minister is the head of the Cabinet and appoints and dismisses the Ministers of State, a majority of whom must be Diet members. Shinzo Abe currently serves as the Prime Minister of Japan.

Source : http://en.wikipedia.org