Nippon Telegraph and Telephone West President Masaaki Moribayashi said Thursday that he will step down at the end of March to take responsibility for massive customer information theft at a subsidiary.

"Our social responsibility is extremely grave. I'm stepping down to take the blame," Moribayashi told a news conference in Osaka, offering an apology.

Last year, NTT West said that a former temporary employee who worked for the subsidiary, NTT Business Solutions, stole customer data and sold it to a list broker and others.

NTT West has been slapped with administrative guidance by the communications ministry for insufficient supervising.

On Thursday, NTT West released a report of investigations into the incident conducted by outside lawyers and others.

According to the report, the NTT West group carried out in-house investigations in 2022 on the possibility of information leakage after a client brought the matter to its attention.

But group officials tasked with the probes concluded without sufficient investigations that there was no such leakage.

Apparently in hopes of continuing transactions with clients, they even made false reports on security measures, saying that encryption software that was not actually used had been installed.

Thursday's report, however, concluded that the officials had no intention to conspire with the former temporary employee or cover up the incident.

To prevent a recurrence, NTT West will increase the staff of its information security division to 100 people and invest about ¥10 billion to upgrade its monitoring system.

The former employee is believed to have been stealing customer information for a decade from around July 2013. The person has been arrested and indicted for violating the unfair competition prevention law.