A tire deflation warning system, including a device for detecting a rotational velocity of a wheel attached to a vehicle and a device for judging a tire pneumatic pressure drop by determining a reduced pressure judging value that judges the tire pneumatic pressure drop made of difference of rotational velocity sum of two pairs of the wheels which are diagonally disposed in the vehicle, correcting the reduced pressure judging value by multiplying a weight coefficient that continuously varies according to the velocity of the vehicle, and comparing the corrected reduced pressure judging value with a threshold value, wherein it is tentatively judged whether or not there is a reduced pressure wheel; and in the case when it is tentatively judged that a driving wheel is being reduced pressure, the reduced pressure judging value is multiplied by the weight coefficient which is larger than in the case when it is tentatively judged that a driven wheel is being reduced pressure.