Kan took the top leadership position under the administration of the now defunct Democratic Party of Japan (DPJ).
From citizens' movement to politics
Kan first won a Lower House seat in 1980 as a member of the party then known as the Socialist Democratic Federation.
He launched his own political career after helping the election campaign of Ichikawa Fusae, who worked to improve the status of women in Japan.
Kan said after his first election victory, "A person with experience in a citizens' movement has taken part in political activities."
Apology to victims of HIV-tainted blood products
In 1996, Kan joined the Cabinet for the first time as health minister under a coalition government led by the Liberal Democratic Party, the Social Democratic Party and the New Party Sakigake.
Kan became known for tackling the issue of HIV infections transmitted to Japanese patients through tainted blood products. He apologized to the victims, saying, "it's the health ministry, the government and the country's responsibility."
Forming a party, becoming PM
Later that year, Kan founded the DPJ with Hatoyama Yukio, who had previously served as prime minister for less than a year from September 2009. Hatoyama said he and Kan were like a traditional Japanese stand-up comedy duo, sharing one spotlight.
DPJ candidates won a large majority in the August 2009 Lower House elections, and Kan became prime minister the following year. On March 11, 2011, a massive offshore earthquake shook Japan, setting off a deadly tsunami which led to a nuclear crisis.
Kan was forced to grapple with a complex and difficult situation, including the meltdowns of three reactors at the tsunami-damaged Fukushima Daiichi nuclear power plant.
However, his government's disaster response came under severe criticism not only from the then-opposition Liberal Democratic Party but also from within the ruling DPJ. Kan stepped down and his Cabinet resigned en masse in August 2011.
Change of power is important
Looking back at his response to the disaster, Kan told a news conference on Sunday that he was aware of various opinions about his visit to the nuclear plant the day after the disaster struck. He said that at the time, he seriously acknowledged "the possibility that people might not be able to live in the Kanto region again."
Kan added that he thinks that a change of power is one of the most important events in a democratic nation. He said the fact that he had not been a member of the Liberal Democratic Party but still had an opportunity to become prime minister suggests Japan's democracy was functioning.
Kan said he would remain involved in politics in some way, when a reporter asked whether he would retire from the political world. He cited the person who influenced him in the past, saying, "for example, Ichikawa Fusae was brought back again after her retirement."