Abstract: Provided is a smart card for communicating with a host computer through a universal serial bus (USB). The smart card includes an internal clock signal generator to generate an internal clock signal, a period detector to detect a period of the internal clock signal and to generate a control code according to the detected period, and a transmission clock generator to generate a transmission clock signal which varies from the internal clock signal according to the control code. The smart card transfers data in sync with the transmission clock signal.