IY Bank, the banking unit of retailer Ito-Yokado Co., said Monday that 4,000 accounts were erroneously double-billed for monthly fees.

Bank officials said the problem was probably caused by human error and not the result of computer trouble. "But we are now looking into the problem," one official said.

IY Bank, jointly owned by Ito-Yokado and its convenience subsidiary, Seven-Eleven Japan Co., operates 24-hour ATMs installed in the stores of each chain.