To issue a warning against an improper (a lowered) air pressure in accordance with a vehicle state and driver's presence/absence, the warning system includes air pressure measuring means for measuring an air pressure of a tire of a vehicle; air pressure judging means for judging whether or not the air pressure is proper; warning issuing means for issuing, if a result of the judgment is negative, a warning; and driver on-board judging means for judging whether or not a driver is on board the vehicle; and driver detecting means for detecting whether or not the driver is in the proximity to the vehicle. If the results of the judgments by the air pressure judging means and the driver on-board judging means are negative and a result of the detection by the driver detecting means is positive, the warning issuing means issues the warning outside the vehicle.