Cosmo Oil Co. said Tuesday it is highly likely that personal data on 923,000 of its 2.2 million registered credit card users as of March have been leaked.
The oil distributor, which revealed in late April that customers' names, addresses and telephone numbers had been leaked, said other data, such as birth dates, gender and vehicle license numbers may have been leaked as well.
Keiichiro Okabe, chairman and president of Cosmo Oil, told reporters that the information was likely stolen by an employee at an outside system development firm during an operational test for a newly developed computer system.
The company is planning to file a criminal complaint or bring a civil suit over the matter.
To take the blame for the incident, Okabe and Vice President Yaichi Kimura will face a 10 percent pay cut and three executives will have their salaries reduced by 5 percent over a three-month period beginning in July.
The data leak came to light in April after several users of Cosmo credit cards received bills from purported debt-collection entities.
The credit cards can be used at Cosmo Oil gas stations across Japan to pay for fuel, engine oil or a car wash.