Cosmo Oil Co. tried to punish an employee who in late April blew the whistle in connection with a leak of personal customer data, sources said Tuesday.

On the morning of April 21, a message appeared on an Internet bulletin board claiming that data pertaining to holders of credit cards issued by the oil distributor had been leaked, they said.

The message said Cosmo Oil executives were "busying themselves covering up the leakage," and urged a swift announcement on the matter by the company to prevent any further leakage.

That afternoon, Cosmo Oil held a news conference to announce that customer data, including names, addresses and telephone numbers, had been leaked.

The company said the leak may involve its entire 2.2 million credit card holders nationwide. The cards can be used at Cosmo Oil gas stations to pay for fuel, an oil change or a car wash.

According to the sources, the company's corporate ethics office issued an internal document the following day accusing the whistle-blower of "lacking a sense of ethics" for "posting important business information on the Internet."

The action violated the company's code of ethics and the perpetrator could be punished by the company or could be subject to criminal prosecution, the document states.

But the company later decided to abandon the notion of punishment because it was unable to identify the whistle-blower, the sources said.

Kazuko Miyamoto, an expert on consumer protection issues who heads the NACS Consumer Research Institute, said the information posted on the Internet served the public interest.

"It's outrageous that the company attempted to find the person who did it for punishment," she said.

Cosmo Oil's public relations office said the company never tried to cover up the leak and does not consider the person who posted the message a whistle-blower who "acted out of (a sense of) righteous anger."

"At the time, the company was gathering information on the leakage so it could publicize it after fully understanding the situation," an official at the office claimed. "But it appears that someone caught wind of this and posted the information on the Internet to take credit for disclosing the fact."

The official added that the internal document was meant to serve as a warning, as various forms of attacks and slandering of employees using their actual names had been appearing on Internet bulletin boards.

Protection bill OK'd

The House of Representatives passed a bill Tuesday to protect whistle-blowers from retaliation by employers.

The bill, which comes amid a spate of corporate misdemeanors that have been exposed recently, cleared the Lower House plenary session with majority approval from the ruling parties.

It bans employers from penalizing employees -- through dismissal, demotion or salary reduction -- for revealing illegal corporate behavior that runs counter to the public interest.

The bill protects employees of private companies, temporary workers, employees of firms that have business relations with wrongdoers, and public servants.

However, it does not protect managers of wrongdoers' business partners, such as subcontractors. The Democratic Party of Japan had demanded their inclusion in a separate bill it submitted to the Diet.

The Lower House also approved a nine-point supplementary resolution accompanying the bill to confirm that it is aimed at expanding the protection of whistle-blowers, stipulating that tipoffs by those who are not subject to the bill's protection will be covered by other laws.