Resona Bank announced Monday it had lost records, including receipts, containing personal information of about 980,000 clients at 27 branches.
Among the branches affected was the one in the Diet building housing the House of Representatives.
The bank said it had not received any reports of illegal use of the information or withdrawals of cash as the records did not contain customers' passwords. However, the information lost did include names, account numbers and transaction details, a bank official said.
If correct, the loss of information on about 980,000 clients would exceed the 960,000 customers affected by information lost by Bank of Tokyo-Mitsubishi UFJ in October.
Details of Resona's problems were discovered at the end of last year when the branches transferred the materials to control centers in Tokyo and Osaka.
As records on transactions using automated teller machines could not be found, the bank spent about six months searching all of its branches, and discovered three types of information--records of ATM use, withdrawal and deposit slips, and copies of tax payments--had gone missing at 27 branches in Osaka, Tokyo and the surrounding areas.