A method for detecting decrease in tire air-pressure on the basis of wheel rotational information obtained from tires attached to a four-wheeled vehicle. The method includes the steps of: obtaining wheel rotational information obtainable from the respective tires, storing the wheel rotational information, and judging decrease in air-pressure of a tire. The step of judging decrease in air-pressure includes the step of adjusting a sensitivity of judgment upon multiplying a specified sensitivity coefficient to the wheel rotational information of the respective wheels of a front axis or those of the respective wheels of a rear axis of the vehicle and of judging a running condition of the vehicle. When it is determined that the vehicle is performing straight-ahead running, the judgment of decrease in tire air-pressure is performed only on the basis of wheel rotational information when performing straight-ahead running. It is possible to maintain detection accuracy of decrease in tire air-pressure and to prevent erroneous alarm.