Resona Bank said Wednesday its Tokyo headquarters has lost a magneto-optical disk containing data on 14,561 deposit holders including their names and addresses.
The data do not include account numbers or phone numbers, the Resona Holdings Inc. unit, based in Osaka, said.
There is no evidence that the disk was stolen and any attempted access by a third party would be blocked by a password, the bank said.
The disk was found to be missing when the bank conducted an in-house inspection in August.
The information on the disk was used for direct mail campaigns. It went missing after being returned from a contractor, Resona said.
Resona apologized for the loss and said it will take steps, including holding training programs, to ensure that customer data is managed strictly.