Machimura was elected to his first term in the House of Representatives in the December 1983 election, and he was re-elected in each election since. He became Minister of Education, Science, Sports and Culture on 11 September 1997, as part of Prime Minister Ryutaro Hashimoto's second cabinet, and became State Secretary for Foreign Affairs on 31 July 1998, in Keizō Obuchi's first cabinet. In March 2000, he became Special Advisor to the Prime Minister, serving under Obuchi and his successor, Yoshirō Mori. On 5 December 2000, he became Minister of Education, Science, Sports and Culture and Director-General of the Science and Technology Agency, before becoming Minister of Education, Culture, Sports, Science and Technology on 6 January 2001. He was the Minister for Foreign Affairs under Prime Minister Junichiro Koizumi from 27 September 2004 to 31 October 2005. His goals included signing a treaty with Russia relations with China and Korea form leader resolving a border dispute, and investigating the whereabouts of Japanese hostages who were kidnapped by North Korean agents during the 1970s and 1980s. He was replaced by Tarō Asō in the cabinet reshuffle that followed the 11 September 2005 election. He was appointed Minister for Foreign Affairs again by Prime Minister Shinzō Abe on 27 August 2007. In 2006, Machimura became chairman of the Seiwa Seisaku Kenkyūkai, the LDP's largest faction. As such, on 14 September 2007, he backed Yasuo Fukuda's bid to become Abe's successor, following Abe's resignation on 12 September. Since 2007, Machimura had co-chaired his faction alongside Hidenao Nakagawa and Shūzen Tanigawa. In Fukuda's government, sworn in on 16 September 2007, Machimura became Chief Cabinet Secretary and State Minister in charge of abduction issues. He was replaced by Takeo Kawamura in the cabinet of prime ministerTaro Aso, which was appointed on 24 September 2008. He was the vice president of the Japan-China Friendship Parliamentarians' Union.
Personal life
On 18 December 2007, Machimura said at an official press conference that he believed in the existence of UFOs. On 1 June 2015, he died after a cerebral infarction at an hospital in Tokyo.