A tire deflation warning system having a means for detecting the rotational speeds of wheels on a vehicle and a means for judging tires with reduced air pressure from the rotational speed information of the wheels including a means for judging whether driving force works to respective wheels or not a means for calculating a vehicle speed when driving force works to respective wheels, a means for comparing the average of the rotational speeds of the driving wheels against the average of the rotational speeds of the following wheels, with the average of the rotational speeds of the driving wheels against the average of the rotational speeds of the following wheels at normal air pressure in running state in which the vehicle has driving force and a means for judging that the air pressures of the all wheels which are mounted on the vehicle are simultaneously reduced, or that the air pressures of the driving wheels are reduced by the result of the comparison.