A wave of customer data losses has been sweeping across Japan, with more than 800,000 such cases being detected at major banking groups and regional banks, companies and financial institutions said Thursday.

Among the cases at banks, which checked the security of personal data at the instruction of the Financial Services Agency, Resona Holdings Inc. said its four subsidiary banks have lost data on about 287,000 customers.

The cases also include 116,000 at UFJ Trust Bank, 61,405 at Sumitomo Mitsui Banking Corp. and 14,000 at Aozora Bank.

The missing customer data had all been stored on microfilm and in documents.

SMBC, the core bank of Sumitomo Mitsui Financial Group Inc., said it is "highly possible" the data were mistakenly "jettisoned" within the bank, and reckoned the possibility of the data being leaked outside or illicitly used was "extremely low."

The banks found the data losses during in-house probes carried out in response to the recent enforcement of a law requiring companies to prevent customer data from being leaked.

Like SMBC, other banks also said the lost data may have been mistakenly discarded, claiming the chance of a leak outside the banks was slim.

Meanwhile, mobile phone operator NTT DoCoMo Inc. said a subsidiary in Tokyo lost a hard disk storing personal information on up to 48,000 people in May.

The lost data include names, telephone numbers and other information filled out by applicants for NTT DoCoMo's services at sales agents in the Kanto-Koshinetsu region in central and eastern Japan from October 2003 to last March.

The subsidiary, which handles payments to agents, realized the loss of the hard disk in late May and has since tried to find it but in vain. It notified police of the loss Monday.