A vehicle-side device includes a vehicle-side control unit that controls transmission of a request signal and performs predetermined control when authentication is successful on the basis of an answer signal, a portable device includes a portable-device control unit that detects reception intensity information of the request signal transmitted from the vehicle-side device and transmits an answer signal based on the reception intensity, the vehicle-side control unit controls a vehicle-side transmission unit so that the vehicle-side transmission unit transmits the request signal including the determination signal having an intensity level different for each section, and the vehicle-side control unit or the portable-device control unit determines whether or not the request signal is a true signal on the basis of a fluctuation in the reception intensity in each section of the determination signal received by the portable device.