Diet of Japan

The Diet of Japan (国会, Kokkai) is the bicameral legislature of Japan and the supreme law-making body of the nation. Established under the 1947 Constitution (enacted May 3, 1947), it consists of the House of Representatives (Shūgiin, lower house, 465 members) and the House of Councillors (Sangiin, upper house, 248 members). Article 41 of the Constitution declares the Diet “the highest organ of state power”—a designation that marked a fundamental departure from the Meiji constitutional order, in which the Emperor held sovereign authority and the legislature was subordinate to the throne. The Diet exercises all legislative power, elects the Prime Minister, ratifies treaties, and approves the national budget.

Structure

House of Representatives

The House of Representatives (Shūgiin) seats 465 members elected for four-year terms, though the house is frequently dissolved early—triggering snap general elections—by the Prime Minister seeking a fresh mandate or responding to a no-confidence vote. Members are chosen through a parallel (mixed) electoral system introduced in 1994: 289 seats are filled via single-member districts (first-past-the-post), and 176 seats via closed-list proportional representation across eleven regional blocs. Voters cast two simultaneous ballots—one for a local candidate, one for a party list.

The lower house holds precedence over the upper house in four constitutional domains:

1. Budget approval: If the chambers disagree, the lower house position prevails automatically after a joint committee period of 30 days.
2. Treaty ratification: Same 30-day automatic override.
3. Prime Minister designation: If the chambers choose different candidates and a joint committee fails to reconcile them, the lower house choice becomes Prime Minister.
4. Ordinary legislation: The lower house may override the upper house with a two-thirds supermajority vote.

The House of Representatives has been reduced in size several times through electoral reforms aimed at addressing urban-rural constituency disparities: from 512 seats (pre-1994) to 480 (2000 reform), to 475 (2014), to the current 465 (2017).

House of Councillors

The House of Councillors (Sangiin) seats 248 members serving fixed six-year terms, with exactly 124 seats standing for election every three years on a staggered cycle. Unlike the lower house, the House of Councillors cannot be dissolved by the Prime Minister, giving it a measure of institutional stability and longer policy horizons. Members are elected through two concurrent systems: 148 seats via prefecture-based districts (magnitude 1–6 seats depending on prefecture population) and 100 seats via a single nationwide proportional representation bloc, which tends to favor smaller parties and single-issue campaigns.

The Councillors can delay or amend legislation, forcing additional deliberation, and serve as a check on lower-house dominance. However, they cannot ultimately block most bills if the lower house deploys its two-thirds override. A “twisted Diet” (nejire Kokkai) arises when different parties control the two chambers, forcing coalition negotiation—a situation that has occurred notably in 1989–90, 2007–09, and 2010–12, complicating governance significantly.

Sessions

The Diet operates through three types of sessions:

• Ordinary session (tsūjō-kai): Convened once per year in January for 150 days (extendable by Diet resolution). This is the primary budget and legislative session; the national budget must pass by the start of the fiscal year on April 1.
• Extraordinary session (rinji-kai): Called by the Cabinet when deemed necessary, or when at least one-quarter of the members of either house formally request one.
• Special session (tokubetsu-kai): Convened within 30 days of a general election to elect a new Prime Minister by majority vote of both houses.

Electoral System and Voting

Japanese citizens aged 18 and older are eligible to vote—a threshold lowered from 20 in June 2016 under revisions to the Public Offices Election Act, adding approximately 2.4 million new eligible voters. The reform was partly motivated by the desire to include youth in potential constitutional referenda, since Article 96 amendment procedures require a public vote. Candidates for the lower house must be at least 25 years old; for the upper house, at least 30 years old.

Turnout in lower-house elections has declined from around 70–75% in the 1960s–1980s to roughly 50–55% in recent decades, reflecting voter disillusionment and the perception of LDP dominance. Upper-house elections typically draw lower turnout still.

Legislative Process

Bills may originate in either chamber, though money bills must begin in the House of Representatives. Cabinet bills (submitted by the executive) account for roughly 80–90% of all legislation passed—reflecting the dominance of the ruling coalition and the bureaucracy (particularly the Finance Ministry and Cabinet Legislation Bureau) in setting the policy agenda. Diet member-sponsored bills are relatively rare and face higher procedural hurdles.

Committee deliberation precedes floor votes in both houses; specialized standing committees (such as the Budget Committee, Foreign Affairs Committee, and Judicial Affairs Committee) are the principal venues for policy debate and minister questioning (shitsumon). Opposition parties exploit committee procedures—filibustering through extended questioning sessions—to delay or publicize controversial bills.

Constitutional amendments require passage by two-thirds majorities in both chambers simultaneously, followed by ratification in a national referendum by simple majority. As of 2026, no amendment to the 1947 Constitution has ever been successfully enacted, despite recurring political pressure—particularly from LDP governments—to revise Article 9 (the war-renunciation clause) and Article 96 (the amendment procedure itself).

Major Political Parties

The LDP has dominated Japanese politics for virtually the entire postwar period, ruling alone or in coalition except during 1993–1994 (when a fragmented eight-party coalition briefly governed) and 2009–2012 (when the Democratic Party of Japan held power). Since 1999, the LDP has governed in coalition with Komeito, whose Buddhist-affiliated voter base provides reliable electoral support in exchange for coalition participation and policy influence on welfare and education issues.

Historical Role

Before World War II, the Imperial Diet (帝國議會, Teikoku Gikai) operated under the Meiji Constitution of 1889 with fundamentally limited powers. It was bicameral—a House of Peers (Kizokuin, comprising imperial family members, nobles, and high-taxpayer representatives) and an elected House of Representatives—but both chambers operated under structural constraints. Article 1 of the Meiji Constitution vested sovereign authority in the Emperor, not the Diet; the military answered directly to the throne through the Chiefs of Staff, beyond parliamentary oversight; and the Privy Council, appointed by the Emperor, could advise against or effectively nullify Diet legislation.

Suffrage was initially restricted to men paying ¥15 or more in direct taxes (approximately 1% of the population), expanding gradually: in 1900 to ¥10 taxpayers, in 1919 to ¥3 taxpayers, and finally to all men over 25 in the Universal Manhood Suffrage Act of 1925. Women gained the vote only under the postwar 1945–46 electoral reforms, exercising it for the first time in the April 1946 general election, in which 39 women won seats.

The postwar 1947 Constitution—drafted by U.S. occupation staff in six days in February 1946 and negotiated with Japanese officials over subsequent months—abolished the House of Peers, replaced it with an elected House of Councillors, stripped the Emperor of political power, and elevated the Diet to the “highest organ of state power.” This represented a fundamental constitutional rupture, transferring popular sovereignty from the Emperor to the people and establishing civilian legislative supremacy over the military.

Related

• Meiji Restoration — the modernization framework that first introduced parliamentary representation under the 1889 Meiji Constitution
• Shinto Religion — State Shinto’s intertwining with Diet politics and ultranationalism before 1945
• Hirohito — the Showa emperor’s reign, spanning the Diet’s transformation from imperial legislature to sovereign body
• Bank of Japan — the central bank whose statutory independence rests on the 1997 Bank of Japan Act passed by the Diet
• Imperial Succession — the succession framework and the 2017 special abdication law enacted through the Diet
• Postwar Constitution & Democracy — the 1947 Constitution that established the Diet as the “highest organ of state power”
• Liberal Democratic Party — the party that has dominated the Diet across both houses for most of the postwar period
• Supreme Court of Japan — the judiciary whose rulings on electoral apportionment repeatedly pressed the Diet to redraw districts