Patent ID: 8907762

Claim:
An electronic key system that performs ID verification communication to verify an ID of an electronic key, wherein the ID verification communication is a wireless communication started by transmission of a request signal from a vehicle and ended by completion of transmission of an ID code signal from the electronic key in response to the request signal, and the electronic key system determines the location of the electronic key in accordance with a received signal strength of a radio wave used for the ID verification communication, the electronic key system comprising: a plurality of reception antennas having different directivities; a measurement signal transmission circuit that transmits a signal strength measurement signal from the vehicle to the electronic key after transmitting the request signal and before receiving the ID code signal; a signal strength measurement circuit that measures a first signal strength, which is the received signal strength when the signal strength measurement signal is received by the electronic key, and a second signal strength, which is a non-reception period signal strength in a vacant time in which the signal strength measurement signal is not received when the electronic key is performing the ID verification communication that is from when the electronic key receives the request signal to when the electronic key sends back the ID code signal in response to the request signal, wherein the signal strength measurement circuit measures the first and second signal strengths at each of the reception antennas; and a control unit that controls operation of the electronic key in accordance with a measurement result of the first signal strength and the second signal strength, wherein the control unit compares each of the second signal strengths measured at the reception antennas with a predetermined reference value used for noise determination, and transmits the one of the first signal strengths measured at the reception antennas corresponding to a second signal strength that is less than the reference value as the received signal strength of the electronic key.