PURPOSE: To obtain a data writing/reading device to an IC card which can prevent the interception of the data signals transmitted and received and can improve the security.

CONSTITUTION: A power supply means 1 is provided to supply the power Vcc and the GND to an IC card 20 together with a clock supply means 3 which supplies a clock signal CLK, a data transmission/reception means 2 which transmits and receives the data signal synchronous with the signal CLK via a signal line I/O, a reset means 4 which supplies a reset signal RST, and a control means 5 which performs the general control of those means 1-4. The frequency of the clock signal supplied from the means 3 varies while the means 2 is transmitting and receiving the data signal. Therefore it is impossible to obtain the correct logical value when the data signal is intercepted on the line I/O.