The characteristic of the suspension is held to be a soft one when the vehicle is running along a flat surface, while it is automatically switched by the switch unit to a hard one when the cycle period T of vertical vibration of the vehicle height becomes near the cycle of the natural vibration of sprung mass of the suspension at the time of running along a road having an irregular surface or jumping a projection, thereby suppressing the resonance of the vehicle body due to external force applied from the road to improve the comfortability.
