Abstract: A data processing system to manage credit card use for a plural number of credit cards and to reconcile credit card statements. The data processing system 1 has multiple POS terminals 21 and cash registers 25, and a personal computer 45. The POS terminals 21 and cash registers 25 can write card usage information to a small, portable storage device represented by an IC card 10 having a rewritable nonvolatile storage unit for storing multiple card usage information records. The card usage information includes credit card number information, payment method information, payment date information, and payment amount information. The personal computer 45 runs a read process to read the card usage information from the IC card 10, and a transaction management process based on the card usage information read by the read process. The IC card 10 can communicate wirelessly with the POS terminals 21 and cash registers 25.