A passive keyless entry device is constructed by a car mounting device mounted to an automobile, a portable device and transmitter-receivers for monitoring a tire pneumatic pressure. The car mounting device has a low frequency signal transmitting section for transmitting a request signal to the portable device and the transmitter-receivers for monitoring the tire pneumatic pressure, a receiving section for a keyless entry for receiving an answer signal responsive to the request signal from the portable device, a receiving section for monitoring the tire pneumatic pressure for receiving the answer signal responsive to the request signal from the transmitter-receivers for monitoring the tire pneumatic pressure, and a controller for forming the request signal and controlling the operation of a door lock mechanism in response to the answer signal and displaying tire pneumatic pressure information.