The Constitution of Japan states that the nation's "highest organ of state power" is its bicameral parliament, the National Diet. The Diet consists of a House of Representatives (Lower House or Shūgi-in) containing 480 seats, elected by popular vote every four years or when dissolved, and a House of Councillors (Upper House or Sangi-in) of 242 seats, whose popularly-elected members serve six-year terms. There is universal adult (over 20 years old) suffrage, with a secret ballot for all elective offices.

The Japanese parliament

The Cabinet is composed of a Prime Minister and ministers of state, and is responsible to the Diet. The Prime Minister must be a member of the Diet, and is designated by his colleagues. The Prime Minister has the power to appoint and remove ministers, a majority of whom must be Diet members. The conservative Liberal Democratic Party (LDP) has been in power since 1955, except for a short-lived coalition government formed from its opposition parties in 1993; the largest opposition party is the liberal-socialist Democratic Party of Japan.