A method for detecting decompression of tires based on the rotational speed of wheels mounted on a vehicle, comprising the steps of: detecting the rotational speed of each of the above-mentioned wheels, determining the speed against ground of the vehicle, calculating the speed information of a fixed wheel from the rotational speeds of said wheels, calculating the judgment value of the wheel position by comparing the speed information of the wheels with the speed against ground of the vehicle, comparing the judgment value of the wheel position with the basis value of the wheel position obtained by preliminarily comparing the speed against ground of the vehicle with the speed information of the wheels at prescribed inner pressure, and judging decompression of tires based on relation between the comparison value of the basis value with the judgment value and a fixed threshold. A region of judging decompression of tires can be broadened and the accuracy of reduced pressure judgment can be improved.