Kenichi Watanabe, the chief executive and architect of Nomura’s takeover of Lehman Brothers’ Asian and European operations, resigned to take responsibility for the scandal, together with Takumi Shibata, the chief operating officer.

They will be succeeded by Koji Nagai, who leads Nomura’s securities unit, and Atsushi Yoshikawa, chief of Nomura’s operations in the United States, the company announced. The management changes were approved by Nomura’s board early Thursday.

“I resign,” Mr. Watanabe said, offering no apology in his opening remarks at a news conference in Tokyo. He said he had started the promised effort to bolster Nomura’s internal controls while continuing to investigate insider trading at the firm.

“Now it is time for a new era with new people,” he added.

Mr. Nagai promised to regain investor trust in the bank. “I intend to reform the company mind-set,” he said.

The resignations are a fresh blow for a brokerage firm that has struggled since snapping up Lehman’s international operations four years ago.

Nomura and the British bank Barclays, which took over most of Lehman’s North American operations, swallowed the businesses at the height of the financial crisis in 2008 in bold, risky bids to bolster their global standings. But now, as Barclays faces an investigation of interest rate manipulation, Nomura is embroiled in its own new scandal: accusations of widespread leaks of privileged information, part of a widening insider trading investigation by Japanese regulators.

After an internal investigation, Nomura acknowledged that employees had leaked information on at least three public offerings in 2010 to favored fund managers, who were then thought to have used stock short-selling to make money on the drop in the share prices of the three companies.

Investors reacted positively to reports of the management shake-up, first reported by the business daily Nikkei, driving Nomura shares up almost 6 percent before its earnings announcement later in the day.

After stock markets closed, Nomura said it had eked out a net profit of 1.98 billion yen ($25 million) in the three months ended June 30, down almost 90 percent from the 17.7 billion yen it reported in period a year earlier, a result that exceeded analysts’ expectations. Nomura’s chief financial officer, Junko Nakagawa, credited a $1.2 billion cost-cutting drive, which she said had been completed ahead of schedule.

The scandal at Nomura, the biggest Japanese investment house, has prompted much soul-searching. Some experts see roots of the scandal in Nomura’s acquisition of Lehman’s European and Asian businesses, which they say was an audacious pursuit of scale and profit that diverted attention from compliance, even as investment banks came under increased regulatory scrutiny.

Lawyers who led the internal investigation described the work environment at Nomura’s equities sales division as one in which “employees would be willing to do anything to meet sales targets,” a mind-set that gained in importance after the Lehman acquisition, which strained Nomura’s finances.

Others within the industry say the recent troubles suggest Nomura, based in Tokyo, had not seriously stepped up internal controls since the 1990s, when the firm admitted to having paid millions of dollars to cover wealthy clients’ trading losses and to having given money to gangsters and racketeers. The financial services minister, Tadahiro Matsushita, said this month that he could not help but think the insider lapses at Nomura “were a matter of routine.”

Whatever the origins, it is clear Nomura has increasingly been fighting on two fronts: to convince global investors that it is a serious contender, and to calm jitters at home over its internal controls.

“If not properly addressed, the insider trading concerns could hurt the standing of the wider Japanese securities industry,” said Tomonori Ito, a visiting professor of international business at Hitotsubashi University Graduate School of International Corporate Strategy and a former banker at UBS.

Nomura’s acquisition of Lehman’s international operations in September 2008 for a nominal fee — at the time a stunning feat by Mr. Watanabe, who was Nomura’s new chief executive — was viewed as the return of Japan’s once-powerful banks to the world stage.

Instead, the takeover overwhelmed Nomura’s management, swelling its global work force by more than 8,000 in Europe and Asia and saddling the bank with exorbitant labor costs at a time of stagnating revenue from trades and investment banking.

Firm insiders say tensions arose as Nomura allowed Lehman’s pay structure — based on high performance and low job security — to coexist with its own moderate salaries, which were based on seniority and high job security.

The Lehman culture of high-risk aggressive use of leverage and rapid decision-making also clashed with Nomura’s hierarchical and conservative approach, insiders say. Nomura found itself reeling from a string of departures of top former Lehman executives, including Jasjit Bhattal, who resigned in January as chief executive of the wholesale banking division.

Though the acquisition was supposed to join Lehman’s vast global client list of hedge funds and other investors with Nomura’s strength in equities and services in Japan, the union has yet to pay off, and Nomura has continued to lose money abroad. Nomura ranks 16th in global investment banking fees this year, with a 2.1 percent share, according to Thomson Reuters; the numbers cast doubt on the future of Nomura’s global operations.

And all the while, there have been signs of governance lapses. In 2008 and 2009, Japanese regulators accused Nomura of two cases of insider trading. In the second case, the firm fired an employee in its mergers and acquisitions department, saying he had divulged sensitive information to acquaintances on deals in which the bank was acting as adviser. Mr. Watanabe apologized publicly at the time and vowed to revamp the firm’s internal controls.

But by Nomura’s own admission, stronger internal controls did not materialize.

A report released June 29 on an investigation led by lawyers appointed by Nomura outlined what they called “systemic defects that would erode confidence” in the firm.

“There were employees who had the incorrect understanding that it would not be a problem to transmit information to their clients concerning public offerings,” the report said.

Under scrutiny are leaks of nonpublic information before offerings in 2010 for Mizuho Financial Group, one of Japan’s largest banks; Inpex, an oil and natural gas exploration company; and the Tokyo Electric Power Company, or Tepco, the operator of the now-destroyed Fukushima Daiichi nuclear power plant.

Nomura employees are suspected of giving tips to fund managers at Chuo Mitsui Asset Trust on the Mizuho and Inpex offerings, and to First New York Securities on Tokyo Electric, the report said.

The offerings were troubled from the start. Tokyo Electric’s share price fell sharply, for example, in the two weeks before the offering was announced on Sept. 29, 2010. The utility subsequently raised almost $6 billion to finance nuclear power facilities, as well as to invest in overseas natural gas and nuclear projects.

According to Nomura’s internal report, an employee in the institutional equity sales division repeatedly contacted an analyst to gauge whether the utility might be considering a public offering.

The employee “engaged in various information activities to obtain inside information” and was frequently in touch by mobile phone with a fund manager at a consulting firm, who took out a short position on Tokyo Electric two days before the share offer was announced, the report said.

The fund manager also tipped off a trader at First New York Securities, who traded on the firm’s own account a day before the announcement, the report said. Both made large profits.

Nomura initiated the internal investigation after the Japanese Securities and Exchange Surveillance Commission accused the firm in March of leaking nonpublic information. The commission is continuing to investigate Nomura as part of a wider inquiry into insider trading that has also taken aim at Nikko SMBC Securities, one of the largest Japanese brokerage firms, and the hedge fund Asuka Asset Management.

Nomura has not been fined or censured by the commission, though action from regulators is expected soon.

On July 3, the financial authorities asked 12 brokerage firms operating in Japan, including top Wall Street firms, to review how they handled nonpublic information. Mr. Matsushita, the financial services minister, has asked the banks to submit reports by early next month.

But the spotlight has remained on Nomura, and the consequences could be sever. This month, Nomura dropped off the list of organizations chosen by the Finance Ministry to underwrite a government sale of shares in Japan Tobacco worth about $6 billion. Other issuers, including the government-linked Japan Housing Finance Agency and Resona Holdings, have also dropped Nomura as an underwriter for planned fund-raising.

Given Nomura’s powerful position in Japan, however, it is unclear how long issuers will stay away. Pension funds, bond issuers, municipalities and even long-term clients like Nippon Telephone & Telegraph marched away after Nomura’s scandals in the 1990s, only to return eventually.

Nomura has had the largest share of the underwriting market in Japan in 13 of the last 14 years, averaging more than a 40 percent share in the last 5 years, according to Mr. Smith at CLSA.

Some analysts have called for tougher regulations and higher fines.

“Even with this management change,” said Mac Salman, head of Japan financials research at Jefferies, “there are still issues which need addressing at Nomura. Investors need to see a focus on profit generation soon.”