Prosecutors have raided the Tokyo offices of Livedoor, the Japanese internet company run by outspoken entrepreneur Takafumi Horie.
Officials gave few details of the raid, but said Livedoor was suspected of violating Japanese securities laws.
Media reports suggested the firm was being investigated on suspicion it had spread false details about a subsidiary in order to boost its stock price.
A Livedoor spokeswoman denied reports of any wrongdoing at the company.
Soaring stock
Livedoor's maverick 33-year-old boss is a well-known personality in Japan.
A university dropout, Mr Horie - who is president and chief executive of Livedoor - shot to fame following separate failed attempts to buy a TV company and a baseball team.
He also unsuccessfully ran in parliamentary elections last year as an independent candidate supported by the ruling Liberal Democratic Party.
Livedoor is one of Japan's best-known internet companies, and offers consulting, telecoms and software development services.
Its stock has soared in recent years as the company has continued to expand.
Prosecutors said they had also raided the offices of separately listed firm Livedoor Marketing.
Shares in Livedoor closed down 0.4% at 696 yen on the Tokyo Stock Exchange, before news of the raids on Monday emerged.
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