There is provided a passive keyless entry system transmitting a request signal at a predetermined time period from a vehicle-side device, transmitting an answer signal when a portable device has received the request signal, and comprising a control unit that performs predetermined control if the answer signal is received or not received by the vehicle-side device. The portable device includes a portable device control unit that detects the intensity of the request signal transmitted from the vehicle-side device, and is adapted such that, if the intensity of a request signal has changed by a predetermined value or more compared with the intensity of a previously received request signal, a normal answer signal is transmitted as the answer signal, and that, if the intensity of the request signal has changed by less than a predetermined value, a simple answer signal that is shorter than the normal answer signal is transmitted as the answer signal.