Takebe, 63, replaced Shinzo Abe, who resigned from the post to take responsibility for the party's poor showing in the last House of Councilors election.
Monday's party reshuffle coincided with a Cabinet shake-up. Takebe's appointment was viewed as the biggest surprise among the changes, according to political commentators.
The Mainichi Shimbun said in its Monday evening edition that Takebe's appointment was a clear indication of Koizumi's resolve to privatize Japan's postal service, the Japan Post. While ruling party politicians are conspicuously divided over the measure, Takebe clearly supports Koizumi's structural reform policies, including the postal privatization.
Former Defense Agency chief Fumio Kyuma, 63, was named chairman of the General Council, while former International Trade and Industry Minister Kaoru Yosano, 66, was given the post of chairman of the party's Policy Research Council.
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