A keyless entry device, comprising: a vehicle-side device provided in a vehicle and including a vehicle-side transmission unit that transmits a request signal, and a vehicle-side reception unit that receives an answer signal; and a portable device including a portable-device reception unit that receives the request signal, and a portable-device transmission unit that transmits the answer signal in response to the request signal, wherein the vehicle-side device includes a vehicle-side control unit that controls transmission of the request signal and performs predetermined control when authentication is successful on the basis of the answer signal, the 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 the answer signal based on the reception intensity, the vehicle-side control unit controls the vehicle-side transmission unit so that the vehicle-side transmission unit transmits the request signal including a determination signal having a plurality of sections with different intensity levels, 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, characterized in that the vehicle-side control unit or the portable-device control unit calculates a reception intensity change rate in each section of the determination signal received by the portable device, and compares the reception intensity change rate with a threshold value to determine whether or not the request signal is a true signal. a vehicle-side device provided in a vehicle and including a vehicle-side transmission unit that transmits a request signal, and a vehicle-side reception unit that receives an answer signal; and a portable device including a portable-device reception unit that receives the request signal, and a portable-device transmission unit that transmits the answer signal in response to the request signal, wherein the vehicle-side device includes a vehicle-side control unit that controls transmission of the request signal and performs predetermined control when authentication is successful on the basis of the answer signal, the 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 the answer signal based on the reception intensity, the vehicle-side control unit controls the vehicle-side transmission unit so that the vehicle-side transmission unit transmits the request signal including a determination signal having a plurality of sections with different intensity levels, 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, characterized in that the vehicle-side control unit or the portable-device control unit calculates a reception intensity change rate in each section of the determination signal received by the portable device, and compares the reception intensity change rate with a threshold value to determine whether or not the request signal is a true signal.