PURPOSE: To enable to automatically regulate the damping force of a shock absorber in accordance with the live load by a method wherein a gas holding sack to change the distance between a car body and an axle is provided and the damping force regulating action of a damping force regulating mechanism is performed by the gas pressure in said sack.

CONSTITUTION: A piston rod 7, a piston 11, the gas holding sack 30, a compressor 82 and the like are arranged between the car body 40 and the axle (not shown). When the live load turns to be larger under the condition that valves 81 and 83 are closed, the sack 30 and the piston rod 7 move toward the direction as indicated with the arrow B, resulting in reducing the volume of gas in a chamber 34 and in lowering the car level of the car body 40. The lowering of the car level causes to operate a detector (not shown) in order to open the valve 81 and to supply the compressed air produced by the compressor 82 into the chamber 34 so as to move the sack 30 and the piston rod 7 toward the direction as indicated with the arrow A, resulting in returning the car level to its former level. In such a manner as described above, the car level is kept at the constant level at all times and the damping force of the damping force regulating mechanism can be automatically regulated.