A tire pressure warning system has an air pressure detecting device 3, a low pressure warning display unit 5 and a fuel consumption deterioration warning display unit 4. The air pressure detecting device 3 detects an air pressure of each tire 2 of a vehicle. The low pressure warning display unit 5 warns an occupant of a reduced air pressure when an air pressure detected by the air pressure detecting device 3 is lower than a running inhibition tire pressure 11 set in advance. The fuel consumption deterioration warning display unit 4 warns the occupant of a deterioration in a fuel consumption when the air pressure is lower than a fuel consumption deteriorating tire pressure 12 which is set higher than the running inhibition tire pressure 11.