Sources say regulators have searched the offices of the Japanese arm of credit card company Visa.
The Japan Fair Trade Commission suspects that Visa Worldwide Japan tried to force card issuers to use its payment service.
The case involves what is known as an "interchange fee." This is an amount paid by a merchant's bank when a customer makes a payment by credit card. The fee goes to the issuer of the card, usually a bank.
The sources say Visa Worldwide Japan urged issuers to choose Visa's payment network service over those offered by its competitors.
They say the company threatened to raise interchange fees if the issuers did not comply.
Visa Worldwide Japan is suspected of violating the anti-monopoly law. The commission plans to seek cooperation from the card company's head office in the United States and a subsidiary in Singapore.