Mizuho Securities Co., a brokerage unit of Mizuho Financial Group Inc., said Thursday it was responsible for the large number of erroneous J-Com sell orders on the Tokyo Stock Exchange that caused overall share prices to plunge the same day.

The mistaken orders on the Mothers market for startups resulted in the trading of 300 billion yen to 400 billion yen worth of shares in J-Com Co., a manpower service firm that was debuting on Mothers the same day, sources said. A total of 421.3 billion yen worth of J-Com stock changed hands during the day.

"We apologize for causing trouble. . . . We are consulting with the Tokyo Stock Exchange on how to handle the issue and will announce what happened after confirming the cause," Mizuho Securities said in a statement.

J-Com's stock started going wild after one order offered to sell about 600,000 shares for 1 yen each, the sources said.

The order, which offered more than 41 times the number of outstanding J-Com shares -- was probably an accidental reversal of an order to sell one share for 600,000 yen, some brokers said.

J-Com initially fetched 672,000 yen, compared with its initial public offering price of 610,000 yen, but plummeted by its daily allowed maximum of 572,000 yen early in the morning.

The stock then rebounded by its daily limit to 772,000 yen on buybacks and remained unchanged the rest of the day amid a flood of buy orders.

The turmoil swirling around J-Com shares triggered broad-based selling on the Tokyo stock market as investors got concerned about the impact of potentially hefty losses on the brokerage firm and the market overall.

The brokerage that placed the order was not known during trading, and Mizuho Securities announced its errors after the market closed.