PURPOSE: To detect the air pressure of a tire regardless of a stopping or running state in a non-contact state, by operating the detection signals of a sensor for detecting the distance between a vehicle for supporting tires and the earth and a sensor for detecting the distance or load pressure between an axle and a car body.

CONSTITUTION: An axle 2 is pivotaly connected to the center of a wheel 1 equipped with a tire being an object to be measured and a suspension unit 4 is provided between the axle 2 and a car body 3. A stroke sensor 5 detects the distance Ls between the axle 2 and the car body 3. As the stroke sensor 5, a position sensor for measuring the change in a resistance value by a slide resistor can be utilized. An earth sensor 6 is secured to the axle 2 and detects the distance Le between the axle 2 and the earth. The sensor 6 can utilize an ultrasonic wave. An air pressure detection system consists of a sensor part 7 consisting of the stroke sensor 5 and the earth sensor 6, an operation processing part 8 for operating detection results and a display part 9 for displaying the air pressure of the tire.