PURPOSE: To enable the contactless transmission of power and data signals on the side of a card by using a data signal derived from a card resonance circuit while connecting a station resonance circuit through an antenna to the card resonance circuit.

CONSTITUTION: A stationally station 1 is provided with a high frequency oscillator 3 for generating a radio frequency signal to be sent to an amplifier/detector 4 connected to a resonance circuit 5. The output of the detector 4 is sent through a band pass filter 6 to a microcomputer 7, and an effective frequency is changed by controlling the output of the oscillator 3. A portable data transaction card 2 is provided with a resonance circuit 11 and its output supplies a DC voltage through a rectifier 12 to a circuit. The circuit 11 is connected through a loading circuit 14 to a data converter 15 and its output is sent to a data memory 16. A reader circuit 17 is connected between the circuit 11 and the data converter 15, and a card data communication circuit is effectively constituted.