Member of the House of Representatives Following his father's announced retirement in 2008, he was elected to his father's former seat representing the
Kanagawa 11th district in the
August 2009 general election, in which many other LDP seats were lost to the
Democratic Party of Japan. He faced criticism for being a hereditary politician. He campaigned in a rented Toyota Prius with a volunteer staff. Koizumi became head of the LDP's young legislators caucus in October 2011, a post previously held by Prime Ministers
Takeshita,
Uno,
Kaifu,
Abe and
Asō. In February 2012, he started a project called "Team 11," which sent members of the division to areas of the
Tōhoku region affected by the
2011 Tōhoku earthquake and tsunami on the 11th of each month to talk to locals and report back on the state of the reconstruction efforts. The group had 82 members, all under the age of 45, as of March 2013. Some observers compared the group to the powerful "Machimura faction" led by
Nobutaka Machimura in terms of its political weight. He was critical of the LDP under party president
Sadakazu Tanigaki. In his first meeting as a party officer, he stated that "the image of the party is that it doesn't listen to the opinions of young people, has old thoughts and a hard head. That is why trust will not be restored." He argued in a November 2011 speech that the party's stance on the controversial
Trans-Pacific Partnership trade agreement needed to be clarified. He later advocated breaking up the LDP's agreement with the
Democratic Party of Japan and
Komeito to pass an overhaul of the social security and tax system, directly arguing to Tanigaki that the party's mission should be to take down the DPJ government and to restore LDP control, and drawing comparisons to his father's maverick reputation. Koizumi broke ranks with the LDP in August 2012 as one of seven LDP legislators who refused to walk out of the no-confidence vote instigated by
Ichiro Ozawa against Prime Minister
Yoshihiko Noda, in which the LDP and NK had agreed to throw out their votes. Although Koizumi voted for the no-confidence resolution, it was ultimately voted down 246–86. He voted for
Shigeru Ishiba against
Shinzō Abe in the LDP leadership election of September 2012, but did not make his vote public until after the election in order to avoid influencing others' votes. Koizumi was re-elected in the
December 2012 general election, which restored LDP control of the government under Abe. In the subsequent
House of Councillors election in July 2013, he focused his campaigning efforts on disaster zones, outlying islands and areas in rapid population decline, giving speeches in support of their local LDP candidates. Kenichi Tokoi, a nonfiction author who wrote a book about Koizumi, said that his goal was to shake as many individual hands as possible and to leave the impression that he was kind enough to visit them, something which he could not achieve by campaigning in big cities. In October 2013, he was appointed Parliamentary Secretary in charge of Tohoku Recovery, in which capacity he would oversee post-disaster reconstruction efforts in
Iwate Prefecture and
Miyagi Prefecture. Ishiba, then secretary general of the LDP, stated that Koizumi "made a very strong case" with local disaster victims "about what he wanted to do and why." Tokoi characterized this posting as a test of Koizumi's administrative ability. On 11 September 2019, Abe appointed Koizumi as
Minister of the Environment. He advocated for environmentalist policies, including ending Japan's use of nuclear and coal power, despite serving in a government considered skeptical of such policies. However, he supports the construction of new
coal-fired power stations in Japan, despite their
particulate and
greenhouse gas emissions. His support for the construction of two coal-fired power stations in
Yokosuka has led him to be a "a target of activists' wrath". In late August 2020, after the resignation of
Shinzo Abe, Koizumi was named as a possible successor. A
Kyodo News survey showed that almost 9% of those surveyed preferred him for prime minister, Koizumi declined to run and endorsed Minister of Defense
Tarō Kōno for the position. After Kōno also declined to run Koizumi and Kono both endorsed
Chief Cabinet Secretary Yoshihide Suga, a fellow Kanagawa politician. Suga ultimately won the LDP presidential election and became prime minister. Suga decided to retain Koizumi in his role. Poor approval ratings and criticism of the government's handling of the
COVID-19 pandemic led to Suga announcing his resignation in September 2021. In the party's leadership election of 2021 (which ended with
Fumio Kishida being elected as leader and later prime minister), Koizumi again endorsed Kōno for the position. When Kishida formed
his first cabinet in October, Koizumi was replaced by
Tsuyoshi Yamaguchi as Environment Minister. From 2021 to 2024, Koizumi remained a
backbencher in the Diet. He was chosen to chair the standing committee on national security of the House of Representatives in January 2024 after the previous chairman resigned in connection to the
2023–2024 Japanese slush fund scandal.
2024 LDP leadership election , who replaced
Fumio Kishida as Prime Minister after winning the
LDP leadership election.|255x255px On 14 August 2024, Prime Minister Fumio Kishida announced he would not seek a second term as LDP President, making the race an open field. By late August, both Kono and Ishiba had already declared their respective candidacies. On 6 September, Koizumi officially announced his candidacy for the LDP presidency. In a press conference he pledged to introduce legislation that would legalize separate surnames for married couples and proposed holding a national referendum to determine whether or not
Article 9 of the
Constitution should be amended. Koizumi promised to dissolve the
lower of house and call a general election “as soon as possible” if elected President and later Prime Minister. His candidacy was endorsed by former prime minister
Yoshihide Suga. Koizumi gave his first street speech the following day in Tokyo's
Ginza district. Koizumi, along with fellow front-runner
Shigeru Ishiba, has been described as one of the "
centrists" of the election. Koizumi lost the election, placing third behind
Sanae Takaichi and Shigeru Ishiba. Ishiba defeated Takaichi in a runoff election, having received support from Koizumi's camp. As LDP president, Ishiba appointed Koizumi chairman of the Election Strategy Committee, a senior party office. In the
October 2024 election Koizumi traveled across the country to rally support for LDP candidates, but the results were unfavourable, with the ruling coalition losing its majority. Koizumi resigned as election strategy chairman to take responsibility. He was replaced by
Seiji Kihara as the party's election chief. Koizumi was subsequently appointed as executive secretary to the LDP political reform headquarters. When asked in February 2025 about the possibility of running for party leadership again, Koizumi didn't rule it out, saying "I will do my best to become a politician that people want to support." Later that month, Koizumi suggested that the
Kansai-based opposition
Innovation Party should join the LDP-Komeito
coalition, amidst budget negotiations that were taking place in the lower house. He continued to advocate for this, stating “we should officially ask for a coalition” in March. Koizumi simultaneously criticized the possibility of a
grand coalition with the
Constitutional Democratic Party, the chief opposition party.
Minister of Agriculture (2025) On 21 May 2025,
Agriculture, Forestry and Fisheries Minister Taku Etō submitted his resignation to Prime Minister Shigeru Ishiba, after stating on 18 May that he had never bought rice, and was instead provided with a free supply of it from his local constituents; the comments were considered to be inconsiderate amidst a nationwide rice
shortage, and reportedly upset Ishiba. Koizumi, having chaired the LDP's Agriculture & Forestry Division and its Comprehensive Fisheries Policy Research Council, was immediately seen as the most likely successor to the disgraced Etō, who had been sacked by Ishiba. Koizumi was appointed Agriculture Minister by Ishiba on the same day. He stated that Koizumi's main objective in the new role would be to stabilize rice prices. Etō and Koizumi had a testy relationship dating back 10 years when the former reportedly told Koizumi, who is 20 years his junior, "I hate you". However the two reportedly became friends after Koizumi invited Etō to dinner at some point afterwards. Koizumi's first official act as Agriculture Minister was to temporarily suspend bidding for stockpiled rice. He also confirmed to reporters that he himself had purchased rice for his family. Upon taking up the new role, Koizumi styled himself as the “Minister in Charge of Rice.” At a speech given at the
Ministry of Agriculture, Forestry and Fisheries the day following his appointment, Koizumi emphasized the importance of reducing rice prices, saying he felt "a sense of crisis" and would seek to restore public trust in the ministry. Koizumi sought to enact swift reforms at the ministry upon his appointment. At his inaugural press conference the previous day, he told reporters that the government would temporarily cancel the fourth bidding process for stockpiled rice that was originally scheduled for later in May, and said he was considering cancelling competitive bidding for the rice in favor of selling it directly to supermarkets and the food service industry through discretionary contracts. Koizumi stated that the goal of the ministry would be to lower rice prices to ¥2,000 per 5 kilograms by June.
2025 LDP leadership election After Prime Minister Ishiba resigned, Koizumi announced his second leadership candidacy in September 2025 for the
2025 leadership election. In opinion polls throughout the election, Koizumi and former Economic Security Minister
Sanae Takaichi were seen as the frontrunners as they both polled in first place various times. On September 24,
Bunshun Online reported possible
stealth marketing by Koizumi's campaign, citing an e-mail by his public relations chief
Karen Makishima giving associates examples of comments to make online to promote Koizumi. Koizumi acknowledged the facts and apologised on 26 September, but denied having been aware of it. Makishima resigned from the campaign. This led to his campaign receiving several death threats and bomb threats, however Koizumi stated his intent to keep running in the election. On 4 October, Takaichi came in first place during the first round of voting with 183 (31.07%) of the vote. Takaichi would go on to defeat Koizumi in the run-off by a 54% to 46% margin.
Minister of Defense (since 2025) , shortly after his appointment as Defense Minister, 21 October 2025 , 28 October 2025 , Koizumi and
Gilbert Teodoro during a multilateral meeting in
Kuala Lumpur, 1 November 2025 On 21 October 2025, Koizumi was appointed as
Minister of Defense by
Prime Minister Sanae Takaichi. At his inaugural press conference the following day, he told reporters that he wants to ease the country's rules on defense equipment exports, and move up the revision of the government's three key defense and security policy documents. On 29 October 2025, one week after becoming Defense Minister, Koizumi held talks with
United States Secretary of Defense Pete Hegseth in
Tokyo. ==Views==