Throughout his political career, Kan has sought to establish an alternative to Japan's powerful conservative Liberal Democratic Party, which dominated politics for half a century before being ousted in August elections.

He earned popularity among the public during his years as a health minister in the 1990s, when he pushed public servants to disclose the ministry's involvement in allowing the use of imported blood products tainted with HIV.

Kan, who will have his hands full as both the finance minister and the deputy prime minister, has criticised the finance ministry as a symbol of old-style politics led by unelected but powerful bureaucrats.

He faces the tough task of steering the world's second largest economy out of its worst post-war downturn, while also keeping the soaring national debt under control.