The 1955 system (Japanese: 55年体制, Hepburn: 55-nen Taisei), 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, 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 Junnosuke Masumi [ja], 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.
