Financial institutions have reported about 6.78 million cases of missing client data, the Financial Services Agency announced Friday.
Including data losses already reported, there have been about 8.62 million incidents of missing information at 424 financial institutions nationwide.
The FSA had asked 1,069 banks, "shinkin" credit banks, credit unions, securities firms and insurance firms nationwide to report on how they handle personal information.
In the latest findings, the data were lost at 287 financial institutions, the agency said, adding that so far, there have been no reports that the missing data were abused in any way.
In the latest findings, data were lost at 277 banking institutions, two brokerages and eight insurers.
In most of the cases, the financial institutions most likely disposed of documents and other records with the personal information by mistake.
Financial services minister Tatsuya Ito said during a news conference after Friday's Cabinet meeting the FSA might take administrative action if any banks were found to be lax in managing the sensitive information.
"We will study the gravity and maliciousness of each incident before determining what action, if any, should be taken," Ito said.
The FSA had told financial institutions to inspect their data management systems and report their findings by the end of June after it was discovered in March that Mizuho Bank and Mizuho Trust & Banking Co. had lost information on tens of thousands of clients.