Officials from Japan's fair trade watchdog have raided Microsoft's Tokyo offices on suspicion of anti-monopoly law violations, authorities said.
A Microsoft spokeswoman confirmed that the Fair Trade Commission inspected the software company's headquarters.
The commission said it believed the US giant had attached unfair conditions when signing software deals with Japanese computer manufacturers.
Microsoft said it was looking into the reasons for the raid.
Multimedia complaints
A spokesman for Microsoft's Japanese subsidiary confirmed the raid but declined to comment further.
"Some commission officials came to our headquarters this morning and they are holding a meeting with our company officials," the spokesman told the AFP news agency.
It is not the first time the US software giant has been accused of abusing its monopoly on PC operating systems to push prices higher or harm rivals.
It is already in settlement negotiations with the European Commission, which has accused the company of trying to stifle competition for multimedia players, by tying its Media Player programme to its Windows operating system.
RealNetworks is suing Microsoft over the same issue, accusing it of unfairly monopolising the growing market for digital music and video.
Market leader
Microsoft has denied such allegations, insisting the market is competitive.
The company last year said it had settled 10 consumer class action suits in a number of US states at a total cost of $1.55bn. The legal cases accused Microsoft of using its dominant position in the market to overcharge for its software.
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