A frequency-dependent mechanism provided at the lower end of a piston rod includes a tubular housing displaceable in an inner tube, together with the piston rod as one unit, and further includes a free piston and O-rings. A shutter of a passage area varying mechanism is provided with an axially extending inner hole, orifices, and oil grooves. The opening area between oil holes 3C and the orifices 33B is changed according to the rotational position of the shutter. The opening area between oil holes 3D and 3E and the oil grooves 33C and 33D and the opening area between oil holes 3F and 3G and the oil grooves 33E and 33F are also variably adjusted according to the rotational position of the shutter.