Born in 1946 in Yamaguchi Prefecture as the son of bussinessman, Kan graduated in 1970 from the Tokyo Institute of Technology, and opened a patent office in 1974. He actively engaged in civic movements for years and achieved a seat in the lower house in 1980 as a member of United Social Democratic Party (USDP). He gained national populality in 1996 when as health minister he exposed the minister's responsibility for the spread of tainted blood. At that time, he was a member of small party forming the ruling government with the Liberal Democratic Party (LDP). His action was completely unprecedented and was applauded in public media. In 1998, his sex scandal forced him to step down from the leader of newly-formed DPJ. After Yukio Hatoyama resigned as the leader of the party, Kan took over the position. In July 2003, the DPJ and the Liberal Party led by Ichiro Ozawa agreed to form a uniformed opposition party to prepare for the general election that was anticipated to take place in the fall.
During the campagin of the election of 2003, the DPJ called the election as the choice of the government between the ruling LDP-bloc and the DPJ and Kan was presented as the alternative candidate to then Prime Minister Koizumi Junichiro. His face was used as the trademark of the campagin againt the LDP.