To provide a method for detecting tire pneumatic pressure drop capable of detecting the simultaneous pressure drop in two front wheel tires or two rear wheel tires.

The tire pneumatic pressure drop detecting method for detecting pneumatic pressure drop in a tire in accordance with rotational information obtained from the tire mounted on a vehicle comprises the steps of detecting the rotational information for each tire, of storing the rotational information for each tire, of detecting whether the vehicle travels on a flat road or not, of detecting whether the vehicle is in driving or braking, of comparing the rotational information for the two front wheels and the two rear wheels of the vehicle when the vehicle travels on the flat road and judged that the vehicle is neither in driving nor in braking, and of judging the simultaneous pressure drop in the two front wheel tires and in the two rear wheel tires based on the relation between the compared value of the rotational information for the two front wheels and the two rear wheels and a predetermined threshold value.