Koike is former newscaster and has also previously served as defence minister and as environment minister in previous governmennts.
Koike will require the support of atleast 20 lawmakers to make her bid for the post in the Japanese Parliament.
Opinion polls show Koike trailing former foreign minister Taro Aso. Political commentators believe Koike will have a tougher job to convince her male colleagues than the people
Also in the race is 70-year-old fiscal policy minister Kaoru Yosano and former defence minister Shigeru Ishiba and former land minister Nobuteru Ishihara both 51, from the Liberal Democratic Party.
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