Confidence From Chief of SoftBank in Sprint Bid

But the next move by Masayoshi Son, SoftBank’s founder and chief executive, astounded corporate Japan. Mr. Son plunged deeper into debt to acquire Vodafone’s Japanese cellphone unit for about $15 billion — Japan’s largest leveraged buyout at the time — to muscle his way into the mobile market.

SoftBank is now one of Japan’s biggest companies — its market capitalization is three times that of Sony’s — and Mr. Son is confident that his plan to break into the American mobile market with a $21.6 billion bid to buy Sprint will be easy.

“We’ve fought far tougher battles than this,” Mr. Son told investors on Friday in Tokyo at SoftBank’s annual general meeting. Hours earlier, a rival bidder for Sprint had confirmed it was ending its pursuit of the mobile network, America’s third-largest. “It’s just been so much easier this time round,” he said.

Most analysts agree that Mr. Son, who seems set to acquire Sprint along with its Clearwire unit, faces long odds as he seeks to compete in the American mobile market. But his record for taking on giant companies — and coming out on top — bodes well for his chances, they say.

“Mr. Son is simply the most talented and audacious businessman Japan has ever produced,” said Kazuo Noda, chairman of the Japan Research Institute. “And when he goes into a new market, he doesn’t just take on the big guys. He changes all the rules.”

It is Mr. Son’s background as an outsider in Japanese society that makes him so willing to take risks, people who know him say, including Mr. Noda, who first met Mr. Son, then a college graduate, three decades ago.

A third-generation Korean-Japanese, a minority group that still faces discrimination in Japan, Mr. Son has spoken of his roots in a slum on the southern island of Kyushu. But after his family’s fortunes improved, he made his way to the United States.

At high school and then at college in California, Mr. Son was dazzled by an entrepreneurial culture that he said he felt had been lost in Japan. And most of all, he was persuaded the world was about to experience a revolution in information technology.

Mr. Son founded SoftBank in 1981 first as a publisher of computer magazines, then as a successful distributor of computer software. Using money from that business, Mr. Son made a series of investments in the early 1990s, including a one-third stake in what was then a little-known American start-up named Yahoo. That bet paid off, giving Mr. Son money to expand his empire as Yahoo’s share price climbed.

But Mr. Son burst into the big leagues in the 2000s, buying a stake in a bankrupt bank, a small telecommunications company, a baseball team and a part of Alibaba of China, now the world’s largest e-commerce company

When SoftBank began offering fast DSL Internet service in Japan in 2001, it took on losses to undercut competitors’ prices by half. Government officials fretted that the price war would cripple the entire market. But consumers loved the sharp fall in prices, and the DSL market grew almost thirtyfold in two years.

Around that time, Mr. Son also started pushing the Japanese government to let SoftBank into the country’s highly regulated mobile sector, dominated by NTT DoCoMo, a subsidiary of the former state telecommunications monopoly. When his demands were snubbed, Mr. Son took the Ministry of Internal Affairs and Communications to court, accusing it of colluding with NTT to shut out competition.

The ministry eventually agreed to talks, but ultimately gave SoftBank a license to an inferior spectrum. Mr. Son, furious, turned to Plan B. In March 2006, SoftBank announced it would acquire Vodafone’s struggling Japanese unit, borrowing 1.2 trillion yen ($10.3 billion at the time) to finance the purchase. A day later, Mr. Son returned his original license to the government.