Japan's stock market regulator said it was probing the country's top brokerage Nomura Holdings but declined to give further details.
In a brief statement on its website on Thursday, the Securities and Exchange Surveillance Commission revealed it had started investigating Nomura, but a spokesman declined to say more when contacted by AFP.
While the watchdog refused to say what the probe was about it comes as it carries out a high-profile investigation into insider trading among top Japanese firms.
A Nomura spokesman, who also declined to comment directly on details of the case, said "it is true that we are under investigation, and we will fully cooperate with... their probe."
The news pushed Nomura shares down 3.48 percent to 332 yen in Tokyo trade.
Last month, Chuo Mitsui Asset Trust and Banking admitted that its staff had engaged in insider trading, with several reports pointing to a Nomura employee as the one who leaked price-sensitive information.
Nomura has declined to comment on those allegations.
The case was tied to a planned share offer two years ago by energy firm Inpex Corp.
Chuo Mitsui sold Inpex shares worth 101 million yen ($1.2 million) about a week before the sale was announced, the regulator has said.
The firm later bought back the shares, reaping profits of about 14 million yen.
Regulators and Tokyo's bourse started a probe several years ago over widespread allegations of insider trading in the shares of Japanese firms.