Microsoft's Japan unit is being investigated by the country's Fair Trade Commission on suspicion of violating anti-trust laws, at a time the US software giant faces similar accusations in Europe.
The commission said today that it believed Microsoft, the world's largest maker of computer software, imposed unfair conditions on computer manufacturers wanting to license its Windows XP operating system software.
It said it hoped to resolve the matter as soon as possible.
A spokeswoman for the unit, Microsoft Co Ltd, confirmed that representatives of the fair-trade body were at its headquarters on Thursday but said she had no further details.
Microsoft Japan does not disclose sales figures, but it said it was the largest revenue contributor in Asia for the parent company.
For the third quarter ended December 31st, Microsoft posted $10.2 billion in revenue.
The investigation is only the latest for Microsoft, which has faced accusations it has abused its dominance of the software market to push prices higher or harm rivals.
It is in settlement negotiations with the European Commission, which says the firm abused its dominant position and curbed competition by tying its Media Player programme - used for playing music and videos - to its Windows operating system.