A variable damping characteristics shock absorber includes a piezoelectric unit which serves as a sensor for detecting piston stroke and an actuator for adjusting stiffness of a flow restriction valve and thus for adjusting the damping characteristics of the shock absorber. The shock absorber includes a pair of flow restriction valves, in which one of valve is active in response to a piston bounding stroke for generating damping force by restricting the fluid flow and the other is active in response to a piston rebounding stroke for generating a damping force by restricting fluid flow. The piezoelectric unit is disposed between two valves in such a manner that it may monitor piston strokes both in the bounding and rebounding stroke directions and adjust stiffness of both of the flow restriction valves.