An error in inputting a stock trade roiled the Tokyo Stock Exchange and is expected to cost the securities arm of one of Japan's biggest financial groups as much as $250 million, in what could become one of the most costly and embarrassing mistakes in the history of the Tokyo exchange.

The error came on the trading debut of J-Com Co., a job-recruiting firm that listed Thursday on the Tokyo exchange with the help of Mizuho Securities Co., the brokerage arm of Mizuho Financial Group Inc.

Early Thursday morning, Mizuho Securities mistakenly placed an order to sell 610,000 shares for one yen each, instead of one share for 610,000 yen ($5,043), the company said at a late-night news conference. The order -- for more than 42 times the number of shares J-Com had outstanding -- sent shares of the tiny company plummeting by their daily maximum price move to $4,729.

That first error was exacerbated by software glitches that prevented Mizuho Securities from canceling the transaction when it discovered the mistake, a minute and a half after it was made. After frantically trying to cancel the order for about half an hour, the brokerage firm was forced to buy back many of the shares at higher prices than it had sold them for, Mizuho Securities said. Mizuho Securities said it was able to buy back most of the shares it agreed to sell, but it wouldn't say how many more it still must buy.

The company has four days to settle the orders. J-Com shares closed Thursday at $6,382.

Mizuho Securities President Makoto Fukuda said the company expects to lose at least $224 million from the mistake, though the loss could exceed $250 million. Still, "given our strong financial standing, we don't think we will have a problem" absorbing the loss, Fukuda said, adding that the company was very sorry for the turmoil it caused in the marketplace.

Rumors of a huge trading mistake spurred selling of shares in financial companies and helped push the Nikkei index down 301.3 points, or 1.9 percent, to 15,183.36. Financial-services firms suffered big declines during the day as market participants speculated which company was behind the abnormal trading pattern in J-Com shares. Shares of Nomura Holdings Inc., for example, fell 3.7 percent, and E-Trade Securities Co. tumbled 5.3 percent.

Mizuho Securities didn't acknowledge that it was behind the mistake until after the Tokyo market closed. Shares of its parent company ended down 3.4 percent.

Mizuho said the incident raises questions about the health of the computerized order-processing system at the Tokyo exchange because the system accepted Mizuho's order even though it was clearly erroneous. The Tokyo exchange had earlier said it was investigating why the problem occurred.

The incident comes a month after an embarrassing breakdown of trading systems at the Tokyo Stock Exchange that halted share trading for almost an entire day.

It was also reminiscent of a technology gaffe at Mizuho Financial in 2002. At that time, a problem in integrating the computer systems of the three banks that merged to form Mizuho shut down many the company's automated teller machines.

Mizuho Financial, whose share price has more than quadrupled over the past two years as the company climbed out of a bad-loan problem, has planned to list its shares on the New York Stock Exchange next year. It isn't clear whether the latest incident will affect the listing plan.

Mizuho Financial owns 82 percent of Mizuho Securities through the wholly owned Mizuho Corporate Bank, while Norinchukin Bank, the huge parent bank for Japan's agricultural cooperatives, holds the remaining 18 percent.

For the last fiscal year, ended March 31, Mizuho Financial reported group profit of $5.85 billion. The group had previously projected a profit of $5.2 billion for the current fiscal year.

Jamie Miyazaki and Kazuhiro Shimamura of Dow Jones Newswires contributed to this report.

A man watches stock prices tumble in downtown Tokyo. The Nikkei fell almost 2 percent after a costly listing error by Mizuho Securities.