An electronic key system has a portable electronic key and a transmitter provided in a vehicle. The key and the transmitter have respective LC resonance circuits including antenna coils that can be electromagnetically coupled. The portable device has a charge capacitor and a diode. The capacitor is charged with a voltage induced by the coil when the LC resonance circuit resonates. The electronic key is enabled to communicate with the transmitter with the charged voltage of the charge capacitor when a built-in battery runs down.