The houses of the Diet are both elected under parallel voting systems. This means that the seats to be filled in any given election are divided into two groups, each elected by a different method; the main difference between the houses is in the sizes of the two groups and how they are elected. Voters are also asked to cast two votes: one for an individual candidate in a constituency, and one for a party list. Any national of Japan at least 18 years of age may vote in these elections. The age of 18 replaced 20 in 2016. Japan's parallel voting system is not to be confused with the Additional Member System used in many other nations. The Constitution of Japan does not specify the number of members of each house of the Diet, the voting system, or the necessary qualifications of those who may vote or be returned in parliamentary elections, thus allowing all of these things to be determined by law. However it does guarantee universal adult suffrage and a secret ballot. It also insists that the electoral law must not discriminate in terms of "race, creed, sex, social status, family origin, education, property or income".
Generally, the election of Diet members is controlled by statutes passed by the Diet. This is a source of contention concerning re-apportionment of prefectures' seats in response to changes of population distribution. For example, the Liberal Democratic Party had controlled Japan for most of its post-war history, and it gained much of its support from rural areas. During the post-war era, large numbers of people were relocating to the urban centers in the seeking of wealth; though some re-apportionments have been made to the number of each prefecture's assigned seats in the Diet, rural areas generally have more representation than do urban areas.
The Supreme Court of Japan began exercising judicial review of apportionment laws following the Kurokawa decision of 1976, invalidating an election in which one district in Hyōgo Prefecture received five times the representation of another district in Osaka Prefecture. The Supreme Court has since indicated that the highest electoral imbalance permissible under Japanese law is 3:1, and that any greater imbalance between any two districts is a violation of Article 14 of the Constitution.
In recent elections the malapportionment ratio amounted to 4.8 in the House of Councillors (census 2005: Ōsaka/Tottori;
election 2007: Kanagawa/Tottori)
and 2.3 in the House of Representatives (election 2009: Chiba 4/Kōchi 3).
Candidates for the lower house must be 25 years old or older and 30 years or older for the upper house. All candidates must be Japanese nationals. Under Article 49 of Japan's Constitution, Diet members are paid about ¥1.3 million a month in salary. Each lawmaker is entitled to employ three secretaries with taxpayer funds, free Shinkansen tickets, and four round-trip airplane tickets a month to enable them to travel back and forth to their home districts.