The , also known as the one-and-a-half party system, is a term used by scholars to describe the dominant-party system that has existed in Japan since 1955, in which the Liberal Democratic Party (LDP) has held by itself or in coalition with Komeito (from 1999 to 2025) a government nearly uninterrupted,Adam P. Liff, Ko Maeda. Electoral incentives, policy compromise, and coalition durability: Japan's LDP–Komeito Government in a mixed electoral system. Cambridge University Press, 2018. https://www.cambridge.org/core/journals/japanese-journal-of-political-science/article/electoral-incentives-policy-compromise-and-coalition-durability-japans-ldpkomeito-government-in-a-mixed-electoral-system/D153792B61853FAC9AF38A20E5076D32 with opposition parties largely incapable of forming significant or long lasting alternatives, other than for brief stints in 1993–1994 and 2009–2012. The terms 1955 system and the one-and-a-half party system are credited to , who described the 1955 system as "a grand political dam into which the history of Japanese politics surge".
The years of Japan under the 1955 regime witnessed high economic growth, leading to the dominance of the ruling party in the Diet, with an undergirded tight connection between the bureaucracy and the business sector. Due to a series of LDP scandals and the 1992 burst of the Japanese asset price bubble, the LDP lost its majority in the House of Representatives in the 1993 general election, which initially signalled the end of the 1955 system. However, the left-wing Japan Socialist Party, the long-time opposition which finally gained a majority, would soon lose much of its support after it decided to form a coalition government with the arch-rival LDP just a year later, leading to the JSP being refounded as the SDP in 1996, and its coalition partner regaining power. The LDP briefly lost power again in 2009 to the now defunct Democratic Party of Japan before regaining it in 2012, retaining power up to the present day. Nevertheless, it lost its majority in the House of Representatives in the 2024 general election, and its majority in the House of Councillors in the 2025 election, forcing it to run a minority government for the first time. The LDP regained its dominant status by securing a supermajority in the 2026 general election.
In the modern day, Japanese politics often take form of a 1955-like system; although the formation of third parties and poles of opposition may be more frequent. Examples include Nippon Ishin no Kai, the Japan Restoration Party, and Kibō no Tō. These parties often take up a more ambivalent stance to the LDP, agreeing to cooperate on some policies. Numerous small groups also fade in and out of the system. The current main opposition to the LDP is the Constitutional Democratic Party of Japan, a re-foundation of the 2017 party which came from liberal Democratic Party splinters.
Background
Pre-1955 multiparty system
After World War II, Japan was controlled by the Supreme Commander for the Allied Powers (SCAP), which aimed at the eradication of militarism and the promotion of democratization in Japan. SCAP therefore issued a series of policies to arrest suspected war criminals which shuffled the political power in Japan. The power of right-wing parties declined in the immediate post war periods due to the purge. The Japan Progressive Party lost about 90% of the seats in the purge, while the Japan Liberal Party lost about 45%. Meanwhile, since many parties on the left were only legalized under the command of SCAP after the WW2, they were barely influenced by the purge. As a result, the Japan Socialist Party led by Tetsu Katayama won the first general election (1947) after the enforcement of the constitution of Japan.
Although at the time no formal regulations about how to form a coalition government existed, there was a consensus among the major parties that a coalition government should be formed to manage the post-war economic problems. Nonetheless, which parties would be included in the coalition took a long process of negotiations. Both a four-party coalition excluding the Japanese Communist Party (JCP) and a three-party coalition excluding both the JCP and the Liberal Party were suggested within the socialists. The Liberal party, led by anticommunist Shigeru Yoshida, showed great reluctance of joining the coalition. At a meeting between Katayama and Yoshida on May 19, 1947, the Liberal Party asked Katayama to "break off with the leftists" in return for its participation in the coalition. Since the Socialist Party had already claimed to officially cut ties with the Communist Party earlier, it refused such demand. The lengthy coalition-building process ended up with a government headed by the Socialist Party, the Democratic Party, and the National Cooperative Party.
The Katayama-led coalition government did not last more than a year due to opposition from both inside the Socialist Party and outside. In order to form a coalition, Katayama had to make concessions which departed from original policy proposals, which further facilitated the split between the leftist faction and the rightist faction within the party. At the same time, policies which Katayama did implement, such as nationalization of coal and mine production, drove the conservatives away.
Yoshida hegemony
After the failure of Katayama cabinet, voters lost confidence in the Socialist Party, which led to the takeover of government by Shigeru Yoshida. In the general election of 1949, the Democratic Liberal Party led by Yoshida won a majority in the House of representatives with 269 seats out of 466 seats, while the Socialist Party won only 48 seats. This was the first majority cabinet in post-war Japan.
Yoshida Shigeru organized five cabinets as a prime minister between 1946 and 1954. The diplomatic, economic, and security policies which Yoshida adopted when he was in power were altogether referred as the "Yoshida Doctrine". These policies remained influential even after him being voted out of the office by a no-confidence motion by the "Yoshida students" who followed his ideology.
The Yoshida Doctrine has three major components:
Japan relies on its alliance with the U.S. for national security. Japan preserves a low level of self-defense capacity. Japan should concentrate on the reconstruction of domestic economy.
The Yoshida Doctrine set the tone for Japan's economic miracle and alignment with the West. However, Yoshida's "one man" leadership and anti-communist stance was criticized and eventually led to the defection of many Diet members from his party to the new Democratic Party, causing his cabinet to resign on December 7, 1954.
Fall of Shigeru Yoshida
The criticisms of Yoshida cabinet mainly focused on three issues:
In 1951, the San Francisco Peace Treaty and the Security Treaty Between the United States and Japan were signed under Yoshida cabinet. The Peace Treaty was signed without the Communist Bloc's presence, and was condemned by the Communist Party for its anti-communist stance. The Security Treaty was criticized by both the Communist Party and the Socialist Party for the risk of remilitarization, while also criticized by the conservatives for putting Japan in a subordinate position, since Japan was asked to provide military bases for the U.S.'s forces. During the fifth Yoshida cabinet, several bills were passed with the strong opposition of the Left Socialist Party of Japan and the Right Socialist Party of Japan. These included: the Strike Control Act, the Mutual Defense Assistance Agreement between Japan and the United States of America, two education acts which restricted the political participation of school teachers, amendment to the Police Act, Self-Defense Forces Act. Among which, the amendment to the Police Act was pushed through with the opposition parties absent from the Diet. The Yoshida cabinet was also involved in two major corruption cases. One is known as the . Masutomi Ito, the director of the financial institution Hozen Keizai Kai, was accused of fraud and suspected of making political bribery, which resulted in the arrests of several conservative politicians. The other is known as the , in which the shipping business and shipbuilding business were proved to be making pay-offs to conservative politicians. Influential politicians in the ruling party such as Eisaku Sato and Hayato Ikeda were suspected as bribe takers. The Yoshida cabinet reacted to this incident by stopping the arrest of Eisaku Sato.
The three controversies led to the gathering of anti-Yoshida forces. On October 20, 1954, anti-Yoshida conservatives formed a coalition party: the Japan Democratic Party. The Japan Democratic Party along with the Left Socialist Party of Japan and the Right Socialist Party of Japan tabled a no-confidence motion against the cabinet on December 6 and won the majority. As a result, Yoshida Cabinet resigned on December 7 after six years of rule.
Establishment of the 1955 System
Merger of the Japan Socialist Party (JSP)
Within the Socialist party, ideological conflicts had long been a problem. The leftists in the party adopted a Marxist ideology, while the rightists leaned towards a socialist welfare nation under a capitalist system. The San Francisco Peace Treaty and the Security Treaty Between the United States and Japan signed in 1951 triggered the final split. The right socialists agreed on the San Francisco Peace Treaty but were against the Security Treaty, due to their constitutionalist stance, while the left socialists were against both treaties due to their exclusion of the Soviet Union. On October 23, 1951, the old Socialist Party officially split into the Left Socialist Party of Japan and the Right Socialist Party of Japan.
After the split, however, the socialists soon realized the necessity of merging into one party to fight against the anti-communist approach taken by the GHQ and the conservatives, which is commonly referred to as the . After the San Francisco Peace Treaty came into effect, influential members from the Right Socialist Party of Japan came back from the purge and boosted the power of the rightists. The Left Socialist Party of Japan also expanded its power with the support of the General Council of Trade Unions of Japan, also known as . Meanwhile, the conservative government was suffering criticisms from the public for the corruption scandals and Security Treaty. Sensing the possibility of success, despite their ideological differences, to take power from the political right and fight against the anti-communist movement, the socialists decided to reunite and formed the Japan Socialist Party (JSP) on October 13, 1955.
One crucial external factor that caused the merger of JSD is Sōhyō. Sōhyō, the abbreviation of the General Council of Trade Unions of Japan, was a trade confederation in Japan which was established on July 11, 1950, soon after the start of the Korean War. It incorporated about 48% of organized workers in Japan. Initially, Sōhyō was the unified body of anti-communist unions, but it soon changed from a centralist stance to a left stance due to the remilitarization tendency of Japan shown in the Korean War. Due to the rationalization strategy taken by the conservative government Japanese workers, particular those who worked in small and median-sized factories, were facing slow wage increase and even dismissals. In response, Sōhyō organized the anti-rationalization campaign and pressured the merger of JSP.
Merger of the Liberal Democratic Party of Japan (LDP)