A vehicle axle-side tube 2 is slidably inserted inside a vehicle body-side tube 1, and a motor "M" is connected inside the vehicle body-side tube 1, and a ball screw nut 3 traveling together with the vehicle axle-side tube 2 is disposed inside the vehicle axle-side tube 2, and a screw shaft 4 connected to the motor shaft "MS" is rotatably threaded into the ball screw nut 3, wherein a linear movement of the ball screwnut 3 is converted into a rotational movement of the screw shaft 4, which is transmitted to the motor shaft "MS", thereby to generate an electromagnetic force against rotation of the motor shaft "MS" and this electromagnetic force is used as damping force for restricting the linear movement of the vehicle axle-side tube.
