PURPOSE: To interrupt the transmission of vibration to a car body or a crew by preventing the damping force from becoming excessive for the vibration over the resonance frequency under a spring.

CONSTITUTION: When stretching, the volume of an upper working chamber 30 decreases, and the volume of a lower working chamber 33 increases, so the working oil within the upper working chamber 30 passes a hole, an oil path 18, a control valve 9, an oil path 20, and a check valve 54, and flows into the lower operation chamber 33. The damping force at this time becomes the product of the area difference being obtained by subtracting the area of a piston rod 17 from the area of a cylinder 29 and the pressure difference across a control valve 9. When it shrinks, the working oil passes a check valve 49 and flows into the upper working chamber 30 from the lower working chamber 33, and the working oil equivalent to the volume of the entry of the piston rod 17 passes a hole, an oil path 19, and a control valve 9, and flows into the oil chamber of a reservoir 1, and pushes up a free piston. The damping force at this time becomes the product of the area of the piston rod 17 and the pressure difference across the control valve 9.