Prime Minister Shinzo Abe on Sunday matched the record of 1,806 days in office set by former Prime Minister Yasuhiro Nakasone in the 1980s, the fourth-longest term in the postwar era.
The figure combines Abe's first short stint from 2006 to 2007, before he resigned over ill health, and his second administration, which began in December 2012.
Chief Cabinet Secretary Yoshihide Suga, who is the top government spokesman and often described as Abe's right-hand man, told a news conference Friday that Abe's longevity in office is the result of "making clear the parameters of what he's going to do and taking political initiative to bring the country forward."
Japan's longest-serving postwar prime minister was Eisaku Sato, who served for 2,798 days in the 1960s and 1970s, followed by Shigeru Yoshida, who served in the 1940s and 1950s and logged 2,616 days in office.
Abe, 62, will match the third-place record of Junichiro Koizumi, from whom he took over in 2006, if he stays in office until May 27.
Abe is the sixth-longest-serving prime minister when prewar and wartime leaders are taken into account, with Taro Katsura holding first place at 2,886 days served in the 1900s and 1910s.
Abe would have to serve until Nov. 19, 2019, to match Katsura's all-time record, something that would only be possible if the Liberal Democratic Party goes ahead with a plan to alter its rule on term limits for party presidents to a maximum of three terms spanning nine years.
There is no limit on how long prime ministers can serve, but by convention they must remain the leaders of their parties.