To cope with battery exhaustion of an electronic key as a portable machine without sophisticating a system configuration.

Both the portable machine 1 and a vehicle-side transmitter 2 have LC resonance circuits, and two antenna coils 18 and 30, which constitute the LC resonance circuits, respectively, are electromagnetically coupled to each other. The portable machine 1 has a diode 21 and a charge capacitor 20, in which an induced voltage generated in the LC resonance circuit of the portable machine 1 is rectified and stored, when the LC resonance circuit of the transmitter 2 performs a resonance operation. Thus, the portable machine 1 can perform transmission/reception operations by using the induced voltage which is stored in the charge capacitor 20.