VEHICLE-USE ELECTRONIC KEY SYSTEM

A vehicle-use electronic key system comprising a control unit (14) mounted on a real car (100A), and a electronic key (12) for sending a response signal (Sa) on receiving a request signal (Sr) sent from the control unit (14) via a transmission antenna (72), the antenna (72) being installed near the middle of the real car (100A). Specifically, when a line segment (wheel base) (154) connecting the center (150a) of a front wheel (150) to the center (152a) of a rear wheel (152) is assumed, the transmission antenna (72) is installed in any location within a range from the upper part to the lower part of the real car (100A) within a range from a 1/4 point (P1) to a 3/4 point (P2) of the wheel base (154) with the center (150a) of the front wheel (150) as a reference.