Electronic key system operable with a charge capacitor power

An electronic key system comprising:

• a portable device having a built-in battery for receiving an interrogatory signal and transmitting a code signal with an electric power of the built-in battery;
• a security control means for transmitting the interrogatory signal to the portable device, receiving the code signal from the portable device and checking whether the code signal is a right one,
• wherein the portable device and the security control means have LC resonance circuits including coils adapted to be electromagnetically coupled each other, respectively,
• wherein the portable device has a power storing means for storing an electric power induced by the LC resonance circuit of the portable device when the LC resonance circuit of the security control means resonates, thereby to receive the interrogatory signal and transmit the code signal with the electric power of the power storing means, and
• wherein the coils of the portable device and the security control means is used as antennas of the portable device and the security control means for signal transmission and reception when the portable device is driven with the electric power of the power storing means to transmit the code signal.