Device and method for monitoring the tire pressure

The device has a transmitter for transmitting a signal to a vehicle-installed device based on air pressure measured by a sensor. The device determines if the air pressure of the tire is abnormal and a recording section records abnormality data in the pressure. When the device communicates with a portable device having a warning indicating section, the device searches for any abnormal data and transmits an abnormality signal. The portable device is part of the driver's keyless entry unit. The portable device indicates a warning signal of an abnormal air pressure of the tire by the air pressure abnormality signal. An independent claim is also included for a method for performing a tire air pressure abnormality warning operation.