To provide a tire pressure drop detecting method capable of detecting decompression of one wheel tire, detecting simultaneous decompression of four wheel tires and improving accuracy of decompression determination.

In the tire pressure drop detecting method, an internal pressure drop of a tire is detected on the basis of rotation information acquired from the tire attached to the a vehicle. It includes a process of detecting rotation information of each tire, a process of calculating a wheel velocity from the rotation information of each tire, a process of storing the wheel velocity of each tire, a process of measuring an absolute velocity of the vehicle, and a process of comparing the measured absolute velocity of the vehicle with the wheel velocity of each tire and determining the internal pressure drop of the tire.