Nomura Securities Co., Japan's largest securities firm, said two of its managing directors may have been trading stocks illegally since March 1995. The executives most likely were secretly dabbling in stocks from a corporate account without the account holder's knowledge, said Atsushi Saito, executive vice president. He did not identify the two, who remain in their posts. He also declined to say how much money was involved in questionable transactions. Japan's Securities Exchange Surveillance Commission has been investigating the matter since September. Saito said Nomura has organized its own investigation and has already found three transactions that it judged illegal. But the company won't take any action against the two executives until the SESC has made a decision, which is expected soon, he said.