Intel, the world's biggest maker of computer chips, has confirmed that Japan's Fair Trade Commission raided its offices there on Thursday.
Intel sets the tone for the tech sector
"We're aware of the fact that the authorities paid us a visit," said Intel spokesman Tom Beerman.
"We're co-operating fully with their investigation," he added.
Intel has declined to comment on the reason for the probe, which media reports say is looking for anti-competitive practices.
Reports say the Japanese competition watchdog is looking into whether Intel sought to persuade its clients not to use central processing units made by rivals such as Advanced Micro Devices (AMD).
AMD Japan has said that the FTC visited its offices too.
"They showed up today and asked a few questions and the meeting has ended already," Reuters news agency cited Shun Yoshizawa, corporate marketing director at AMD Japan as saying.
"The FTC is doing this on suspicion that Intel might have broken an anti-trust law. That's the only thing we know," said Mr Yoshizawa.
Intel's sales for 2003 were $30.1bn (£16.3bn), up 13% from $26.8 billion in 2002, and it made full year net income of $5.6bn.