The present invention provides a communication system that enables safe and easy addition of a function for use of credit card settlement services to a portable information terminal. In this system, if a user brings his credit card close to the portable information terminal, the portable information terminal reads a card ID stored in a contactless IC card provided in the credit card. Then, the card ID is transmitted to a card management system which checks whether or not the card ID is valid. If confirming that the card ID is valid, the card management system instructs the portable information terminal to write the card ID thereto. If the user places an order of a merchandize, the card ID written to the portable information terminal is provided therefrom to use credit card settlement services.