Social Democratic Party 社会民主党 | |
|---|---|
 | |
| Japanese name | Shakai Minshu-tō |
| President | Mizuho Fukushima |
| Founded | 19 January 1996; 30 years ago (1996-01-19) |
| Preceded by | Japan Socialist Party |
| Headquarters | 2-4-3-7FNagata-cho,Chiyoda-ku,Tokyo 100-0014 |
| Ideology | Social democracy[1] Democratic socialism[2] Progressivism[3] Pacifism[4] |
| Political position | Centre-left[5][6] toleft-wing[7][3] |
| International affiliation | Socialist International[8] |
| Colours | Sky blue[9] |
| Councillors[10] | 2 / 248 |
| Representatives | 0 / 465 |
| Prefectural assembly members[11] | 6 / 2,614 |
| Municipal assembly members[11] | 81 / 28,940 |
| Website | |
sdp | |
TheSocial Democratic Party (社会民主党,Shakai Minshu-tō; often abbreviated to社民党Shamin-tō;SDP) is apolitical party in Japan that was established in 1996.[12] Since its reformation and name change in 1996, it has advocatedpacifism and defined itself as asocial-democratic party.[13] It was previously known as theJapan Socialist Party (日本社会党,Nihon Shakaitō; abbreviated to JSP in English).
The party was re-founded in January 1996 by the majority of legislators of the former Japan Socialist Party, which was the largest opposition party in the1955 System. However, most of those legislators joined theDemocratic Party of Japan after that. Five leftist legislators who did not join the SDP formed theNew Socialist Party, which lost all its seats in the following election. The SDP enjoyed a short period of government participation from 1993 to 1994 as part of theHosokawa Cabinet and later formed a coalition government with theLiberal Democratic Party under 81stPrime MinisterTomiichi Murayama of the JSP from 1994 to January 1996. The SDP was part of ruling coalitions between January and November 1996 (First Hashimoto Cabinet) and from 2009 to 2010 (Yukio Hatoyama Cabinet).
In the2019 Japanese House of Councillors election, the party won four representatives in theNational Diet, two in the lower house and two in the upper house. In November 2020, the party entered into a merger agreement with theConstitutional Democratic Party (CDP) with the SDP's members in the Diet caucusing with the CDP.[14] The party president Mizuho Fukushima held her seat and, in the2022 House of Councillors elections, the party cleared the minimum two percent voter share to maintain its legal political party status.[15]
In 1995, the former Japan Socialist Party (JSP) was in a deep crisis, facing criticism for entering a coalition with its long-time rival, the LDP and for core policy changes.[16] Aiming at saving the party, the leadership of JSP decided to dissolve the party and to establish a new social democratic party. In January 1996, a new party, the Social Democratic Party, was established, along with the dissolution of JSP.De jure, JSP changed its name to the Social Democratic Party (SDP) as an interim party for forming a new party, and a movement for transforming the SDP into a new social-democratic and liberal party was unsuccessful.
Under Murayama's successorRyūtarō Hashimoto (LDP), the SDP remained part of the ruling coalition. Long before its disappointing result in the1996 Japanese general election, the party lost the majority of its members of the House of Representatives, mainly to predecessors of theDemocratic Party of Japan (DPJ) that was formed in 1996, but also some to the NFP and other opposition parties. After its electoral defeat in the 1996 general election, when it lost another 15 of its remaining 30 seats in the lower house, the SDP left the ruling coalition, which it had entered as the second-largest force in Japanese politics, as a minor party.
The SDP won six seats in the2003 Japanese general election, compared with 18 seats in the previous2000 Japanese general election. The party's opposition to the Self-Defense Forces reverted to the abolition of the forces in the long term, the policy it had in the 1950s. Doi had been the leader since 1996, but she resigned in 2003, taking responsibility for the election losses.Mizuho Fukushima was elected as the new party leader in November 2003. In the2004 Japanese House of Councillors election, the SDP won only two seats, having five seats in theHouse of Councillors and six seats in theHouse of Representatives. In 2006, the party unexpectedly gained the governorship of theShiga Prefecture. In the2009 Japanese general election, the DPJ made large gains and the SDP maintained its base of 7 seats in the, becoming a junior partner in a new government coalition; however, disagreements over the issue of theFutenma base led to the sacking of Fukushima from the cabinet on 28 May and the SDP subsequently voted to leave the ruling coalition.[17]
As of October 2010, the SDP had six members in the House of Representatives[18] and four members in the House of Councillors.[19] Following the2012 Japanese general election, the party retained only six seats in the whole of the Diet, two in the House of Representatives and four in the House of Councillors. The count lowered to five seats in 2013. In 2013, the party's headquarters in Nagatacho, where the party's predecessor the JSP had moved in 1964, were demolished. The headquarters moved to a smaller office in Nagatacho.[20]
During the nomination period of the2016 Japanese House of Councillors election, the party signed an agreement with theDemocratic,Communist andPeople's Life parties to field a jointly-endorsed candidate in each of the 32 districts in which only one seat is contested, thereby uniting in an attempt to take control of the House from the LDP/Komeito coalition.[21] The party had two Councillors up for re-election and fielded a total of 11 candidates in the election, 4 in single and multi-member districts and 7 in the 48-seat national proportional representation block.[22]
In the2017 Japanese general election, the party managed to hold to its two seats it had prior to the election.Tadatomo Yoshida declined to run for re-election when his term expired in January 2018.Seiji Mataichi was elected unopposed in theensuing leadership election and took office on 25 February 2018.[23][24]
On 14 November 2020, the party voted to agree to a merger arrangement with theConstitutional Democratic Party of Japan (CDP), allowing members to leave the SDP and join the latter party. The majority of the party supported the agreement and joined the CDP; however, party leader Fukushima herself was opposed to the merger agreement and remains a member of the Social Democratic Party.[25]
In the2021 Japanese general election, the party lost one of their two seats.
On 2 November 2025, Kunio Arakaki expressed his intention to quit the Social Democratic Party for a "different option and disagreements about expanding the party's influence".[26] Arakaki finally quit the party on 19 November 2025, making the party lose all of its seats in theHouse of Representatives.[27]
After the CDP and the Komeito formed theCentrist Reform Alliance in January 2026, the SDP objected to of the alliance's acceptance of the 2015Legislation for Peace and Security. The SDP then withdrew from the CDP parliamentary faction to form its own.[28]
Party policies include:[13][29]
| No. | Name (Birth–death) | Constituency / title | Term of office | Election results | Photo | Prime Minister(term) | ||
|---|---|---|---|---|---|---|---|---|
| Took office | Left office | |||||||
| Preceding party:Japan Socialist Party(left-wing) | ||||||||
| Chair of the Social Democratic Party (1996–present) | ||||||||
| 1 | Tomiichi Murayama (1924–2025) | Rep for Ōita 1st | 19 January 1996 | 28 September 1996 | - |  | Hashimoto1996–98 (coalition, confidence and supply) | |
| 2 | Takako Doi (1928–2014) | Rep for Hyōgō 7th | 28 September 1996 | 15 November 2003 | - |  | ||
| Obuchi1998–2000 | ||||||||
| Mori2000–01 | ||||||||
| Koizumi2001–06 | ||||||||
| 3 | Mizuho Fukushima (b. 1955) | Cou for National PR | 15 November 2003 | 25 July 2013 | - |  | ||
| Abe S.2006–07 | ||||||||
| Fukuda Y.2007–08 | ||||||||
| Asō2008–09 | ||||||||
| Hatoyama Y.2009–10 (coalition until 30 May 2010) | ||||||||
| Kan2010–11 | ||||||||
| Noda2011–12 | ||||||||
| Abe S.2012–20 | ||||||||
| － | Seiji Mataichi (1944–2023) (acting) | Cou for National PR (until 28 July 2019) | 25 July 2013 | 14 October 2013 | - | |||
| 4 | Tadatomo Yoshida (b. 1956) | Cou for National PR (until 25 July 2016) (29 July 2019 - present) | 14 October 2013 | 25 February 2018 | 2013 Tadatomo Yoshida – 9986 Taiga Ishikawa – 2239 2016 Unopposed |  | ||
| 5 | Seiji Mataichi (1944–2023) | Cou for National PR (until 28 July 2019) | 25 February 2018 | 22 February 2020 | 2018 Unopposed | |||
| 6 | Mizuho Fukushima (b. 1955) | Cou for National PR | 22 February 2020 | Incumbent | 2020 Unopposed | .png) | ||
| Suga2020–21 | ||||||||
| Kishida2021–24 | ||||||||
| Ishiba2024–25 | ||||||||
| Takaichi2025–present | ||||||||
| Election | Leader | Seats | Position | Constituency votes | PR Block votes | Status | ||||
|---|---|---|---|---|---|---|---|---|---|---|
| No. | ± | Share | No. | Share | No. | Share | ||||
| 1996 | Takako Doi | 15 / 500 |  16 | 3.0% | 5th | 1,240,649 | 2.19% | 3,547,240 | 6.38% | LDP-SDP-NPS coalition |
| Opposition | ||||||||||
| 2000 | 19 / 480 | 4 | 4.0% |  6th | 2,315,235 | 3.80% | 5,603,680 | 9.36% | Opposition | |
| 2003 | 6 / 480 | 13 | 1.3% | 5th | 1,708,672 | 2.87% | 3,027,390 | 5.12% | Opposition | |
| 2005 | Mizuho Fukushima | 7 / 480 | 1 | 1.5% |  5th | 996,008 | 1.5% | 3,719,522 | 5.49% | Opposition |
| 2009 | 7 / 480 | | 1.5% | 5th | 1,376,739 | 1.95% | 3,006,160 | 4.27% | DPJ-SDP-PNP coalition | |
| Opposition | ||||||||||
| 2012 | 2 / 480 | 5 | 0.4% | 8th | 451,762 | 0.76% | 1,420,790 | 2.36% | Opposition | |
| 2014 | Tadatomo Yoshida | 2 / 475 | | 0.4% | 6th | 419,347 | 0.79% | 1,314,441 | 2.46% | Opposition |
| 2017 | 2 / 465 | | 0.4% | 7th | 634,770 | 1.15% | 941,324 | 1.69% | Opposition | |
| 2021 | Mizuho Fukushima | 1 / 465 | 1 | 0.2% | 8th | 313,193 | 0.55% | 1,018,588 | 1.77% | Opposition |
| 2024 | 1 / 465 | | 0.2% | 10th | 283,287 | 0.52% | 934,598 | 1.71% | Opposition | |
| 2026 | 0 / 465 | 1 | 0.0% | 11th | 148,666 | 0.26 | 728,601 | 1.27% | N/a | |
| Election | Leader | No. of seats total | No. of seats won | No. of National votes | % of National vote | No. of Prefectural votes | % of Prefectural vote |
|---|---|---|---|---|---|---|---|
| 1998 | Takako Doi | 13 / 252 | 5 / 126 | 4,370,763 | 7.8% | 2,403,649 | 4.3% |
| 2001 | 8 / 247 | 3 / 121 | 3,628,635 | 6.63% | 1,874,299 | 3.45% | |
| 2004 | Mizuho Fukushima | 5 / 242 | 2 / 121 | 2,990,665 | 5.35% | 984,338 | 1.75% |
| 2007 | 5 / 242 | 2 / 121 | 2,634,713 | 4.47% | 1,352,018 | 2.28% | |
| 2010 | 4 / 242 | 2 / 121 | 2,242,735 | 3.84% | 602,684 | 1.03% | |
| 2013 | 3 / 242 | 1 / 121 | 1,255,235 | 2.36% | 271,547 | 0.51% | |
| 2016 | Tadatomo Yoshida | 2 / 242 | 1 / 121 | 1,536,238 | 2.74% | 289,899 | 0.51% |
| 2019 | Seiji Mataichi | 2 / 245 | 1 / 124 | 1,046,011 | 2.09% | 191,820 | 0.38% |
| 2022 | Mizuho Fukushima | 1 / 248 | 1 / 125 | 1,258,502 | 2.37% | 178,911 | 0.34% |
| 2025 | 2 / 248 | 1 / 125 | 1,217,823 | 2.06% | 302,775 | 0.51% |
Up for re-election in 2031
Up for re-election in 2028
In alliance with the centre-left Social Democratic Party, the Hatoyama government had the majority necessary to push its radical programme through, over - riding any LDP opposition in the Upper House.
社民党の新垣邦男衆院議員（沖縄2区）が2日、沖縄県宜野湾市で記者会見し、離党する意向を表明した。「党勢拡大を目指すとの思いでいたが、（党内の）見解の相違やかみ合わないところがあった」と述べた。
Fukushima, 62, a former lawyer, feminist activist and member of parliament's upper house, is deputy head of the small opposition Social Democratic Party. She has a history of representing and advising victims of sexual harassment.
This article incorporates text from this source, which is in thepublic domain. Country Studies.Federal Research Division.
Bracketed numbers indicate numbers of seats in theHouse of Representatives (Lower House) of theNational Diet immediately after the2026 general election | |||||
| Officially recognized political parties |
| ||||
| Political organizations with seats in theNational Diet |
| ||||
| International | |
|---|---|
| National | |
