The current Japanese constitution was promulgated in the year 1946 during the occupation by the Allied powers:
The Japanese parliament is called the Diet. It consists of the House of Representatives (480 members) and the House of Councillors (242 members). The members of the Diet are elected by the Japanese people.
The cabinet is headed by the Prime Minister. The cabinet further consists of the ministers which are appointed by the prime minister and are usually members of the Diet. The prime minister is elected by the Diet.
The highest court is the Supreme Court. Other courts are district courts, high courts, family courts, and summary courts. Judges are appointed by the cabinet.
The minimum voting age is 20 years. Women received the right to vote in the new constitution. Elections for the House of Representatives are carried out every four years, and half of the House of Councillors is elected every three years. Beside the national elections there are prefectural and municipal elections.
For a much more extensive outline of national and local government, see this Wikipedia article: