PURPOSE: To accurately and easily execute cancel processing or the like for a credit transaction by previously storing the transaction details data of credit transaction in a portable customer's data storage medium, reading out the stored contents at need, executing the cancel processing of the credit transaction, preparing transaction cancel data, and transmitting the prepared data to a credit company.

CONSTITUTION: An IC card to be a portable customer's data storage medium can be used also as a credit card and includes an ID file set up with the ID data of its owner and a transaction details file for successively storing transaction details data generated by a POS terminal 7. When a credit transaction is executed, a POS terminal in a POS system generates its transaction recording data and stores the data in the transaction details file of the IC card. When the customer requests the return, exchange, etc., of a commodity, specific transaction details data are read out from the transaction details file of the IC card and register minus processing is executed.