PURPOSE: To allow change in spring constant and damping force for better travelling comfort and stability by interconnecting gas chambers of main and auxiliary actuators through a gas passage and providing in the gas passage a switch valve whose opening and closing can be controlled.

CONSTITUTION: A main actuator 1 includes a cylinder 3 having inside thereof an oil chamber 5 and at the lowermost end thereof a fixture 4 for attaching the actuator to an axle. An auxiliary actuator 2 is divided by a free piston 30 into an oil chamber 31 and a gas chamber 32. The gas chamber 32 and the gas chamber 12 of the main actuator 1 are interconnected through a gas passage 33 and, a switch valve 34 is provided in the gas passage. At normal straight driving on a smooth road, the valve 34 is open, allowing communication between the gas chambers 12, 13 to lower the spring constant, and an orifice 19 for adjusting damping force is open to reduce the damping force for improved travelling comfort. At turning or others, the valve 34 is closed to raise the spring constant, and the orifice 19 is closed to increase the damping force for better stability of the vehicle body posture.