An electronic key system determines that legitimate communication has been performed on the basis of conditions including that a measured distance from a ranging request unit to a ranging response unit measured on the basis of a ranging request signal and a ranging response signal is within a threshold range that is set on the basis of the distance from a communication area of a transmission antenna where an electronic key is determined to be located by a key location determination unit to one of either the ranging request unit or the ranging response unit that is provided in an on-board device.