A method of judging a vehicle loading condition includes detecting rotational speeds of wheels mounted on a vehicle, calculating a judging value of tire pressures based on the difference of the sum of rotational speeds of two pairs of wheels located at the opposing corners of the vehicle and detecting an acceleration to a lateral direction of the vehicle. The loading state of the vehicle is judged by the change of a primary coefficient based on an approximated relation between the acceleration to a lateral direction and the judging value of the tire pressures. A tire deflation warning system detects the stop of the vehicle, and the air pressure of four wheels is judged not to be decreased if the loading condition of a vehicle is changed more than a fixed value before and after a vehicle stop. The tire deflation warning system can prevent a deflation alarm for a vehicle with a heavy load.