A detection method of reducing the air pressure of tires based on the rotational speed of wheels mounted on a vehicle. The method includes steps of: detecting the rotational speed of each of the wheels; storing the rotational speed of wheels; determining a judgment value of decompression of tires based on the relative comparison of the rotational speed; storing the judgment value in a non-volatile memory; determining an average value obtained by a moving average of a certain number of the judgment value; and judging decompression of tires by comparing the average value with a threshold. Decompression of tires can be detected even if the traveling of a vehicle is repeated for a short time and the chance of judging the reduced pressure of tires is magnified to be able to enhance the safety of vehicle traveling.