An air suspension system which includes an air spring provided between a car body and a suspension arm for adjusting the car height by changing the volume of the air spring. The air spring includes a diaphragm and an air piston formed to contact with the diaphragm. The air piston has a plurality of wall portions arranged longitudinally. Each wall portion is different in its external shape from the portion adjacent thereto. The external shapes of the air piston includes a combination of at least two shapes of the group including a straight cylindrical shape, a tapered shape converging downward and a tapered shape diverging downward.