New director general of the IAEA, Japan's Yukiya Amano, is seen after his election in Vienna, Austria Thursday, July 2, 2009.

A Japanese diplomat who favors a firm approach toward Iran was elected to lead the International Atomic Energy Agency on Thursday, narrowly edging out a South African for the post.

In the last of six rounds of voting at the organization's headquarters in Vienna on Thursday, the Japanese diplomat, Yukiya Amano, won the required two-thirds majority of the 35-nation board, with 23 votes in favor and one abstention, said an agency official who spoke on the condition of anonymity because the election results will not be official until the governing board formally accepts them on Friday.

In earlier rounds of voting, Mr. Amano had been locked in a tie with a South African diplomat, Abdul Samad Minty.

Mr. Amano, 62, will become the fifth director general of the agency, an organization affiliated with the United Nations that monitors and investigates charges of illicit nuclear proliferation and promotes atomic energy for peaceful purposes. He will replace Mohamed ElBaradei, whose term expires in November.

Mr. Amano will take the post as the agency faces many challenges to its mission of containing the spread of nuclear weapons, including Iran's efforts to enrich uranium, a recent nuclear test in North Korea and an agency investigation into whether Syria has covertly sought to build its own nuclear capacity.

Depicted by experts as the candidate favored by the United States and other wealthy nations, Mr. Amano favors maintaining the current approach toward controlling nuclear proliferation in Iran, which Western countries suspect of trying to build nuclear weapons. Iran says its nuclear program is purely for civilian purposes to generate energy.

"He's a nonproliferation and disarmament guy, and he believes in it," said David A. Kay, a former I.A.E.A. official and a senior fellow at the Potomac Institute for Policy Studies. "He has been around in trying to keep the inspections in Iran going, and I expect him to continue very much in that line. He will not want to create a situation in which military action is the only alternative."

In a February interview with Reuters, Mr. Amano praised President Obama's readiness to talk to Iran about its nuclear ambitions, saying Iran should be treated with respect through dialogue. The recent postelection turmoil, however, has put those negotiations in question.

The choice of candidates reflected a division in the I.A.E.A. between the Western and industrialized nations that lead the nuclear club and define the atomic agency's prime role as a watchdog, and developing countries more interested in the broader use of nuclear energy.

The director general serves a four-year term. Dr. ElBaradei, 66, has held the post since 1997, when he succeeded Hans Blix of Sweden.

Alan Cowell contributed reporting from Paris.